I love bg's, unmatching and matching alike.  I 
think if it's got the matching bg it's very 
special, but there's nothing wrong about putting 
a non matching bg with a cel.  In my opinion it 
enhances the cel greatly.  I dont like to just 
see white behind the character.  The cel was made 
to be in front of something to create a scene, so 
adding a bg helps that to become so.  All my cels 
have backgrounds, and only two are matching.  
And...what's neat about it is you can take your 
favorite cel and add your own background to make 
your own scene that looks better than the real 
scene!  ^_~