For the most part, sleeping-bag manufacturers prefer you store bags in a loose stuffsack. In the case of down bags, long-term compression can crimp the framework of the down plumules, and theyre less apt to reform to full size. For synthetic bags, imagine a piece of paper folded and held in place. The fold becomes impossible to remove. A similar effect can happen to the fibers in a synthetic bag. So I would tend to discourage you from leaving a stuffed bag in your pack.
So what to do? I spent a number of years in a mountain rescue group and had a similar problem. For the most part, we had a little warning if there would be a guaranteed overnight, and we usually had to repack at that time anyway to accommodate the extra gear (you know, the search-and-rescue mantra is hurry up and wait"). But on plenty of occasions it was a matter of just toughing it out minus a sleeping bag. I always carried a bivy bag for an emergency shelter, and that, plus the clothing I carried, got me through a few miserable nights.
As I see it, the options are threefold. One: Simply keep the sleeping bag in its storage sack near your ready-to-go gear, so when the pager goes off you can take a minute or two to stuff it into the pack. Two: Buy an inexpensive but adequate bag, such as MECs Habanero (from C$82; www.mec.ca), and more or less sacrifice it. Three: Buy a synthetic bag that claims its crush-proof, such as Wiggys SuperLight (US$175; www.wiggys.com). The bag has a proprietary synthetic fill called Lamilite, which Wiggys says wont break down. It offers a lifetime warranty to back it up, so what do you have to loose? And the bag is perfectly warm and stuffable.
Check out this years more than 400 must-have gear items, including a comprehensive sleeping bag section, in the 2006 Buyers Guide.