I know there have been some disagreements about 
this in the past, but I have no questions about 
it:  my framed cels experience WAY more line 
fading than my cels in celbooks.  This was with 
UV-resistant glass (clear), minimal artificial 
light, and no direct sunlight.  I only have about 
three cels framed now and I made sure they aren't 
my favorite ones.  :P

The extent of line fading may differ depending on 
the type of ink used, and this varies from studio 
to studio.  If you really want to frame your cels 
to enjoy them, then go ahead!  But be cautious 
about it if you want to preserve the cel over 
time...and never, EVER frame a Toei production 
studio cel.  

-Anne