I definitely keep my cels in their bags, seperate 
from their sketches and/or backgrounds.  I've been 
a collector on and off for the past 4+ years, and 
in the beginning I remember making the mistake of 
leaving the layers together (or even placing what 
I thought at the time, was a protective layer of 
paper behind the cel), only to have them adhere 
together.  They definitely *can* become re-stuck 
after time, even cels that are several years old 
from older series, and cels that are stored 
properly.  My opinion is that it's safest just to 
seperate. ^_^