I've built sets from the '50s to the '80s and I guess I'm really on a sliding scale being a bit more lenient with the older/more expensive cards. I realize that may be opposite of some folks that might want the major cards in a given set to be the shinning stars.
In general, I:
- Look for cards with good eye appeal, particularly on the front;
- Centering should be at least 70/30 and definitely don't want any card without a border showing;
- No diamond cuts
- Don't mind wrinkles or even small creases that are hard to see with the naked eye;
- Generally don't mind small marks on the backs of cards;
- Always look for good registration. The TPGs don't seem to take this into account as much as other flaws, but I can't stand a blurry photo;
- I'm OK with EX corners; especially on older cards. Like wrinkles on an older person's face - they just belong there. No dog ears though;
- No trimmed cards;
- Print marks are OK, if not too large/distracting and not on a players face;
- Centering on the reverse is not so important unless it is really bad.
Fortunately, for most Topps sets, there are plenty of cards out there. It may just take some time to find one that fits your liking and price point.