When I frame cels I always take them out of the
bags.  Two reasons mainly  - polyethylene bags
have a bit of a milky color to them and make the
cel colors look dull, and if the cel is in a frame
it doesn't need a bag anyway (as the bag is mainly
to protect it from elements like fingerprints, and
the frame serves that purpose).

Polypropylene bags are clearer, but I don't really
see the need of putting a cel in a bag in a frame
as long as the framing is done to archival
standards (buffered matting and the like).

--
Matt Martin -=- cels@nekomusume.net
http://cels.nekomusume.net/