If a card is altered, any number grade, except debatably 1, is incorrect, and the card is less valuable than an unaltered card in that number grade.
Humorously, PSA's own website states "Cards that have been trimmed have very little value." (
link if you want to read for yourself, or copy for posterity) However, as was suggested in other thread, that line may be quietly removed at some point in the near future.