You might want to join the Anime-Beta forum community, which has been talking about conservation issues for some time now. It requires an invite, but that's just to keep spambots out: send me an email and I'll get you one.
There are several members who feel strongly that framing art is a safe and valid solution to storage, and others (like myself) who opt for the online scan + upright binder solution. A good intro to the conservation issues is this thread:
http://www.anime-beta.com/phpBB3/viewtopic.php?f=61&t=29581&p=345176&hilit=library+storage#p345176
I'll just reinforce what I think are the most prudent measures:
1. Handle cels infrequently.
2. Include microchamber paper or some other water/vapor absorber in the storage space.
3. Keep away from light and in a stable temp./humidity environment.
4. Check cels every 3 months for signs of trouble and air them out once a year.
That's based on good library science practices and should keep cels in good condition indefinitely. I have received some with vinegar syndrome issues but (thank heavens) have never had one develop such a problem under my watch.