You might look at Kaijufan's post three threads 
above this one, or do a forum search on the 
merits of UV glass and acid-free mats. :o) It'll 
give you a lot of good detail.

The short of it is going to be that objects are 
always degrading. From the moment of their 
creation, they're going to start deteriorating. 
There's nothing you can do to keep these items 
from deteriorating over time, but you can slow 
down the rate so you can enjoy them for as long 
as possible.

The long of it--do a Google search on the "Agents 
of Deterioration". Some of them (ie, insects) 
aren't probably going to be an issue with cels. 
Many of them-- improper temperature, 
light/radiation, moisture, etc-- will be issues. 
Sunlight = heat, especially in a closed frame. 
Even if the glass filters out the UV rays to keep 
it from fading that way, if the heat builds up 
under the glass, it's going to damage your cel. 
Likewise, if there are smokers in your house, 
that may have an impact on your cel. Those are 
the sorts of things you're going to have to worry 
about, not just with the framing, but also with 
the displaying.

Ultimately, the cel is yours to do with as you 
please. It's up to you to decide whether you want 
to risk it by putting it on display and enjoying 
it that way, versus having it tucked away in a 
dark closet. You can do whatever you want with 
it. The answer to your question is going to be 
yes, no matter what precautions you take, putting 
your cel on display is going to degrade it over 
time... maybe a year, maybe 5 years, maybe 
longer. But if you want to take as many 
precautions as possible, take it to a real framer 
(not a craft store framer) and ask their advice.