I'm usually eternally short of cel bags. But when 
it's possible, I *always* put the cel in the book 
while still in its bag. Otherwise, you risk the 
cel sticking to the paper backing in your cel 
book page - maybe it would also be at more risk 
for getting damaged if it slid around in the page 
pocket without a bag. . .

Many Sharp Smiles,
--Drac