...putting cels on non-matching backgrounds (which 
I see a lot).

Neither type of background belongs with the cel 
(though a non-matching BG is worse, IMHO), but 
both are added to improve appearance.

Eh?

JP

PS--Why are non-matching BG's worse?  It's hard to 
explain.  I guess because it's harder to tell that 
it doesn't belong.  I see animation cels as art 
that represents a piece of a cartoon.  When you 
start mixing in other shows and elements, it takes 
away from that art.  If, however, you are simply 
looking for a prettier picture, I guess it makes 
sense.  (But if that's the case, why not save 95% 
and buy a poster!?!!)  ^_^