Actually I've had a cel come to me which had a 
very slight crease (not in any painted areas 
thankfully).   I put it in a cel book and just 
really didn't think much about it after - I was 
pretty disappointed it arrived damaged, but 
didn't want to try and claim any kind of 
insurance for it since I still was happy enough 
with the cel.  Anyway... :)   When it came time 
to streamline my collection a bit, I took it out 
to rescan so I could notate the crease when I 
auctioned it - and it was gone! ^_^   I think it 
really depends on how bad it is, and if it 
affects painted areas. If the acetate is really 
creased *bad* it might not ever go away =/   but 
if it's just kind of "warped" then you might get 
lucky. :)  Good luck!