I bought a few cels from a dealer at a con that 
were pretty scuffed up from everyone sifting 
through the cel books. When I had collected 
enough cels to need a book, I was extremely anal 
about having them slide around. (The book itself 
is oversized) However small the risk, I didn't 
want to aquire any of those cloudy scuff marks. 
Also I was afraid if I didn't somehow anchor the 
cels, they might slide to the center of the book 
when the pages were turned and consequently get a 
curved bend in them. 

The very ugly result is that I bag and rebag the 
cels. One protects, and other is achored to the 
book to prevent movement. Personally, when I'm 
ready to display a cel, I frame it. But not 
everyone likes to frame considering their is an 
element of risk involved. In any case, I'd say 
unless you have a book that fits your cels very 
precisely, I'd make sure they are protected. If 
you buy some nice clean bags, I bet the slight 
distraction or dulling of colors will more than 
compensate for maintaining the cel's quality.

-MeganD.