Stamps, quality and grading: Quality of cancel / postmark
Most stamp catalogs do a somewhat decent job in displaying the various quality levels of cancelled stamps. However, IMO there are surprisingly large international variations to what is desirable / non-desirable.
The beauty of SOTN
Being from Scandinavia, I’ve been raised to keep postmark as the most important quality factor used stamp can possess. If possible, my dream team of stamps would contain only SOTN (Socked-on-the-nose) items such as these:

2003 Finland - Moomin stamps with a SOTN cancel. These self-adhesive stamps were issued in a booklet.
And to make things even more challening, some serious minded philatelists have come up with a set of rules that “A true SOTN” stamp should qualify:
a) postmark should be in straight / almost straight position compared to stamp. If it’s sideways (or diagonal), then it’s not a a true SOTN.
b) Both date and location should be fully (or as fully as possible) readable from the postmark.
c) the postmark should be as close to stamps issuing year as possible.
Meaning a stamp cancelled a decade (or more) after the issue date, can’t be qualified as true SOTN.
d) Machine cancellations (especially if wavy lines are visible) are not qualified as true SOTN.
This is an difficult condition to collect, especially if following the above rules strictly.
Some countries, like Switzerland or Denmark, are somewhat easy to acquire in with very nice cancellations. But some, like US or UK, are guaranteed to cause lots of gray hairs (if not any already).

US postage stamps with nice SOTN postmark are somewhat hard to come by.
The non-desirable cancellations
On the opposite end of the pool, there are plenty of not so desirable alternatives.
First on the line there are VERY heavy and smudgy postmarks that hide away most of the stamps original design. This was the way original “killer postmarks” (used in some countries on the 19th century) were meant to be to prevent re-use. Sadly, modern day has it’s equivalents that exist not because they kill the stamp, but due to lower running costs compared to alternatives.

Straight from an exchange lot I received yesterday... A killer cancel on 18th century US stamp; these can be quite attractive though they do kill the stamps design. On the right a modern day US spray cancel.
Even more undesirable grade are used, but non-postmarked stamps. These are something I think nobody prefers, but sadly they are becoming more and more common.

2009 USA. No postmark on these stamps. These are best left on cover.
Pen cancellations are a complex issue, as depending on the era (and use/purpose) of stamp, these can be either desirable (first picture below) or non-desirable (second picture below). In both cases, these are samples of their times:

1888 Bavaria - crayon / blue pencil was used instead of normal postmark on some specific cases (such as heavy matter / parcels).

1974 US - some stamps from the UPU centenary set with ugly pen cancellation
And finally there is the case of CTO-cancelled stamps… These are usually visually attractive cancellations, but as they are not “real” a lot of collectors qualify these equal to utter junk.

Eastern Europe countries produced a great number of CTO stamps.
The other quality levels
Though I consider myself somewhat quality conscious, I’m not too worried by the fact a very large part of my collections is somewhere in the between of these two ends.

A sample of average quality cancellations.
In fact, I’m having a blast, as this allows me to work on two frontiers at the same time. While looking for new items, I’m also constantly looking for better quality (especially postmark wise) replacements for items I already have.
Re your SOTN stamps from Finland shown as the first examples, are you collecting stamps or postmarks???
I like some stamps from Finland, and I like to see the stamp design, without having ANY postmark obliterating most of the design, whether it is SOTN or wavy line. Far better, IMHO, is a part cds on one corner.
Fortunately there are plenty of these about for the rest of us as so many people seem to want SOTN !
Thanks for leaving the nicely cancelled stamps for us!
That is a good question Ian…I think that in this matter beauty is largely in the eye of the beholder. It is stamps I collect, but I do too value a good postmark as well.
I have nothing against a corner CDS or wavylines on stamp, but I simply prefer having a postmark with readable location and date on my stamp. This is something I (and lot of collectors of the same generation) was “raised” to value. In similar style on the 1970’s collectors were raised to value never hinged over hinged stamps I believe. So possibly calling it “evolution of the hobby” would do the honors, LOL.
I don’t think there is a “right way” or “wrong way” to collect either. In my only used collection (Germany) I look for a cancellation that best compliments the individual stamp’s design. Sometimes it is a SOTN, sometimes a corner cancel and sometimes a clear impression of one of Germany’s special occasion postmarks.
That’s what makes this hobby great – and what gives us valuable trading material!
I don’t think I’s a difference between the generations but between the countries. I think in some countries like the USA most collectors prefer “lightly cancelled” stamps with only a smal part of thje cancellation in one corner of the stamp, while in other countries like Germany STON is preferred and (modern) stamps without readable date and location or stamps with machine cancellations showing even only a small part of the “wavy lines” killer are considered almost valueless.
I don’t know the reason for this. Maybe it’s the different culture, but maybe it’s just the fact that in countries like the USA SOTN are very hard to find because hand cancellations ae almost never used on normal mail and the machine cancellations are often ugly spray cancellations.
But of cause there’s no “wrong way”, like Greg said. Everyone is free to collect what he (or she) likes.
I agree there are huge countrywide differences… Just yesterday I was (re)reading preface of Scott catalogue and my eyes popped out when I noted a small note that stated “favour cancelled copies sell at large discount”. I admit my initial reaction was WTF??? I guess this explains why items that are easily worth 100€ in Northern European markets can be purchased for few bucks in US markets (and vice versa, as lots of European collectors are not that keen with centering with US stamps). LOL.
But in the end the only thing that matters are personal preferences – not what “official guides” recommend.