As long as the layers are not stuck, I see nothing
wrong with carefully removing the tape and
removing the extra piece.  This is for your
enjoyment of the cel.  If you don't like the way
it looks, I see no problem in doing this.  Just
store the extra piece in a separate bag so you
have the complete cel.  I've done this before with
my own cels and I don't think I've done anything
wrong in doing so.

Just my .02!  :-)

jINx