Sorry, I meant to reply to the OP, not WildBackpacker.

I use a Sea-to-Summit Ultra-Sil dry bag for my sleeping bag, not to compress it (you don't want to do that) but to be absolutely sure it stays dry. I tested the dry bag (in the bathtub) to make sure it didn't leak and continue to test it every spring (some of these bags have been reported to leak in the past, but evidently Sea-to-Summit read those reviews and solved the problem).

If you instead use a pack liner to keep your pack contents dry, then just shove the sleeping bag into the bottom of the liner. Be sure to inspect the pack liner daily for holes. If you don't use either, then get either a pack liner or a dry bag. Stuff sacks and pack covers are NOT waterproof (stick one in the bathtub and you'll find out) and it is vitally important to keep that sleeping bag dry! Even the most experienced of us have been known to slip and fall when fording a creek!

You can save on pack weight by using as few stuff sacks as possible. The ones you do use should be the lightest weight you can find. Most commercial stuff sacks are heavy (coated nylon and a cord closure thick enough to hold a horse)! A number of the "cottage" manufacturers sell stuff sacks made of silnylon, spinnaker or even cuben fiber that also use a very thin cord and tiny cord locks to save weight. The few I use I've gotten from Joe Valesko's ZPacks. The silnylon stuffsacks made by Eureka and sold by Campmor are not quite as well-made, but are reasonably priced.