[glchen]

As other have said, there are many definitions of alterations in the hobby. Here are a few that I can think of:

Soaking a card glued onto something else like a scrapbook: Acceptable
Soaking and pressing a card to remove wrinkles: Not Acceptable
Trimming a hand cut card such as a strip card: Acceptable
Trimming an oversized factory cut card: Not Acceptable
Erasing a pencil mark from a card using a standard eraser: Maybe?
Erasing a pen/ink mark from a card using chemicals: Not Acceptable
Adding color to a card: Not Acceptable
Rebuilding corners: Not Acceptable
Re-backing a skinned card: Not Acceptable