The framer is right and wrong. The framer I worked with mentioned that he seals things in to prevent spiders/insects from getting in. (mats+wood=happy insect home). Also, rag is best, but there are numerous other acid-free/archival grade stuff.
It's probably fine to leave it sealed; just make sure to have an air-out schedule.
In terms of messing things up: You owning the cel is likely 10000000times better for the cel than it rotting in a dealer's or studio's shelf. Do remember that :).
It's your call to keep it framed or not; here's my anecdote:
I framed one of my favorites, and it kept it that way for a year. No damage/deterioration in that time.
What I found was this:
->One awesome cel framed
->Bunch of other cool cels in books
->Kept looking through cel books
->Made books organized just the way I like
->Framed cel kinda got forgotten
I found I enjoy flipping through my cels like pictures; open the book, follow some sort of progression, remember that episode...then put it back on the shelf. Then a few months later...repeat. Seeing the cel every day framed lost the excitement of turning the page of an Itoya and seeing that I-can't-believe-I-own-this cel :). That might be just me though.
I'd keep it framed for as long as you enjoy it that way; however, after some time, bag and binder it for long term storage.
I find these acid-free boards:
http://www.amazon.com/LIFE-Magazine-Backing-Boards-Count/dp/B0034YRN9U/ref=sr_1_4?ie=UTF8&qid=1435713680&sr=8-4&keywords=magazine+boards
Plus some cel-sized clearbags:
http://www.amazon.com/gp/product/B001XE767M?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00
(Assuming it's a 9x10 cel)
Works pretty well. There's sticky stuff on the back of the bag that attaches to board; cel stays straight, cel doesn't touch the board, and the bag is open for air (snip corners if need be). Put somewhere dark or in an Itoya.
A final note: Enjoy your cel now, and understand it may not always look as bright or clear; it's still your cel, and it's still from a show you care about. It was created to be enjoyed; it's a cel's #1 purpose in life...so enjoy it :)