it depends on the studio.  Someone else on here, 
I don't remember who... but s/he and I posted a 
couple times about this.  

I re-bag mine, always, and clip the corner. When 
I DIDN'T clip the corner, I noticed tiny tiny 
spots on my cels starting to fade!  I've clipped 
them now and they've been "breathing" for a few 
months in their cel books... and there hasn't 
been any more fading as of yet.  I collect Ayashi 
no Ceres from Studio Pierrot.  

I am not sure about Sailor Moon and Toei.  :(

But I DO clip mine.  I know Nichibeianime clips 
theirs, and they do have a few SM in their 
personal gallery, I believe!

Hope this helps!
Kristina