I noticed as I was replacing my cel bags last time that 
several of the "cheaper" bags (thinner, softer, the kind 
that stretches when you try to pull tape off it) were all
wrinkly and warped at the point of contact between the cel
paint and the bag. 

Why is this? The cel paint exudes gas/moisture, right? I 
suppose that's a good reason to never seal a cel bag (i.e.
with tape)?