I'm new to cel collecting too, but I keep mine 
out of the bag. With the books I use, the 
dividing paper inside the sleeves is acid-free 
(along with everything else)... and it allows the 
cel to breath.

I believe the reason why most cels get stuck to 
sketchs, backgrounds, etc in the first place is 
because they were still partially wet during 
filming... and afterwards they get shoved with a 
sketch and then put into a stack which compresses 
them down... 

Like I said, I'm new to cel collecting too so 
this might be totally wrong, but I'd think that 
by the time the cel gets to you, the paint has 
dried completely and there shouldn't be a problem 
with it sticking again. (I still wouldn't suggest 
putting a lot of weight on them though because 
who knows ^_^)

Jennifer