Actually I do not believe I am "mondo" 
exaggerating about the importance of -not- 
stacking cels like a sandwich. Stacking cels was 
commonly done in the studios themselves when the 
anime was in production. The animators, paint 
checkers, cameramen and studios themselves 
typcially didn't care how the cels were 
handled/stored after they've completed the 
project (cels are difficult to store and are 
considered hazardous waste). However, as 
collectors we do CARE about the condition of the 
cels we so revere and this is the huge difference 
in priorities. Continuing to stack cels is a 
asinine way to preserve and reduce the damage to 
the deterioration of the cels (you're doing the 
exact opposite). In addition to the dougas 
normally becoming stuck, you also often encounter 
paint loss as well. Who here enjoys seeing a 
otherwise nice cel missing a portion of paint? 
Sometimes it cannot be prevented b/c of the years 
of neglect, but when it is avoidable why not make 
the effort?

As for changing cel bags, from my experience it 
is important. This is true regardless if you cut 
corners of the bags (when you first get the cel) 
to allow the fumes to be released. And I will 
agree I too have seen cels sold in really 
yellowed, wrinkled bags which probably weren't 
changed. But, when I've examined the condition of 
these cels, often I find the paint is cracked. 
It's possible there is little to no connection 
between the cel bags itself and the paint 
cracking/chipping (maybe something else caused 
it), but I've rarely seen it with well-maintained 
cels (ie stored in new cel bags in a study 
portfolio that's not stacked in a cool place). 
Lastly, most of the older dealers do make the 
effort to change their cel bags at least once, 
when the acquire the cel and prepare it to be 
sold. Making the effort to change a cel bag isn't 
terribly difficult and it beats the alternative 
of touching the actual cel directly with oily 
hands.