I have some experience in this; mostly from my own research and some help from a family friend who worked in framing...note that this is ultra-paranoid/conservation...take with a grain of salt:
I can't think of any immediate damage that could happen in the worst of framing jobs; most framing issues are mid-term and long term (eg, cel condition after months or years)
Anyway, the "Best" way to mount a cel is with edge strips. I have framed a cel like this, and it works great:
http://www.arclight.net/~pdb/nonfiction ... -cels.html
I'd definitely get matting between every layer and the backing; you don't want any part of a layer touching anything. Partly because paint sticking, and partly to increase air-flow in case of Vinegar Syndrome. Also, if you can get 8 ply museum rag, that stuff rocks. It's twice as thick as normal matting and stays really flat, which can be important for larger cels.
(As an aside, some cheap acid-free mats are only acid free on one side. Ensure it's acid free all the way through)
On things tape:
Corner pockets or taping the corners is very standard, but can make cels unhappy in the long run. The problem is that if the tape is holding the cel really tight, then the cel is under constant stress pulling across its surface; as the cel naturally expands/contracts with temperature/humidity, it could hurt the paint. Obviously the tape needs to be acid-free/archival safe.
If the tape is not tight, the cel will "bend", which could cause cracks as the paint gets more brittle with age. Likewise, moving the frame will cause motion in the cel. Obviously, this is a bit of a catch-22...
The only good compromise using tape is if there is a single piece of tape at the top and nowhere else, in which case it works well. The cel stays lightly clamped between two mats and can expand as it needs, although it still has tape stuck to it. Obviously it can move a bit if you're moving the frame around too much, as there is very little preventing motion.
The edge strip method solves everything by creating channels around the cel that hold the entire edge; there's no single high pressure point. Likewise, these channels allow for natural expansion of the cel but still hold the cel in place so it does not bend or warp.
Personal opinion:
No way in heck would I let tape touch a raw cel. Acid-free tape holding the mats together, acid free backing, 8-ply museum rag, and museum glass/plexi-UV, and 100% cotton acid/lignen free edge strips lightly clamping the edges of the cel.
kidding 
...even if you replace it later, having a sheet around is really useful for other photography purposes.