Alright, after a brief Staff Discussion, I would say that the position on edits (or, uses as CO's, for that matter, I suppose) is as follows:
If you want to edit a tileset, ALWAYS try to contact the original author. When they cannot be reached, and you are in the honest belief that your edit is not "violating" the probably intended vision of the tileset (i. e. an overall rather minor change), make absolutely sure you give proper credit when using/editing their stuff.
This, of course, doesn't apply when the author explicitly, publicly stated that their tileset is not to be used/edited.