In the interest of tying up the loose ends, and in case this thread can be useful going forward...
PSA's Grading Criteria are online:
http://www.psacard.com/services/psa_...tandards.chtml
BGS's Grading Criteria are online:
http://www.beckett.com/estore/helpsy...x?ArticleId=47
On the topic of centering, this is from the BGS website:
Pristine 10
Centering: 50/50 all around on front. 60/40 or better on back.
Gem Mint 9.5
Centering: 50/50 one way, 55/45 the other on front. 65/35 or better on back
SGC's Grading Criteria are online:
http://www.sgccard.com/grading_scale.htm
On the topic of centering, this is from the SGC website...
Grade Quality Description
100 PRISTINE A "virtually flawless" card. 50/50 centering, crisp focus, four sharp corners*, free of stains, no breaks in surface gloss, no print or refractor lines, and no visible wear under magnification.
98 GEM 10 55/45 or better centering, sharp focus, four sharp corners*, free of stains, no breaks in surface gloss, no print or refractor lines, and no visible wear. A slight print spot visible under close scrutiny is allowable if it does not detract from the aesthetics of the card.