Any good framer you take it to should be able to
cut a mat that will show the cel to its best 
advantage, so you should not need to cut the 
background.  If you live in the San Jose area, I 
can recommend a good framer with animation cel
experience. :)  As for keeping it in the bag, I 
would not.  The bag will be an extra layer between
the cel and the glass and could obscure the image
or distort it by warping.  As long as the framer
uses UV protection glass and all acid free 
archival materials, you should be good to go.  
Also, remember to hang it out of the light.

Jen