I don't really know, but I always do rebag any 
new cel I get.  It's clear that the paint does 
react with the bags, making them pucker, so I 
figure it's a good idea.  It would be useful if 
someone sorted out the fact from folklore on this 
front: should we "let our cels breathe" by 
snipping a corner on the cel bags, or is this 
just a superstition?