......Between the last layers of my b/g cels I do 
just to store them. Theres a lot of pressure from 
other cels in the book if you lay them flat     
(although, there is a keen school of thought 
about hanging the books)
and I dont want the last layer of cel rubbing or 
pressing against the background. 
  Background paint is powdery like watercolor 
paint..Cel paint is sticky and glossy, so the cel 
paint tends to rub against the background and 
make little bare spots in the powdery water color 
paint. 
  Also,a darker colored background paint can 
stain through the lighter colored cel paint, I 
have an unfortunate Ranma 1/2 cel that really 
should have had protection between the last layer 
of cel and the background. You can see the dark 
brown of the background coming through in small 
spots on his face.

LIke I said I do this only to store them, I scan 
all of my cels lately outside of the bags and 
then put them all together again when Im done, it 
gives them a nicer look ( I still have a lousy 
scanner)

It shouldnt make the cels too fat to store.  a 
single bag layer probably wont take too much more 
room and really helps me sleep at night. Cel 
paranoia is an awful thing.
 

Pale