While stapling a cel to a background not 
uncommon, it's not standard procedure either.  
Some people prefer to leave things as is 
(preserve it as it comes, worried about messing 
something up) and others prefer to remove them 
(worried about staples causing tears or rust, 
don't like the centering).  If you are going to 
remove them, I suggest you go about it very 
slowly and carefully.  I also suggest you 
separate your background from your cel with a cel 
bag, regardless of the staple.  Of course, the 
standard disclaimer, I take no responsibility for 
what you do with your cels. ^_^

Best of luck!

Keys