I never use the stuff sack that comes with a sleeping bag. First of all, they are usually heavy. More important, the sleeping bag has to stay dry "at all costs." It's your main source of insulation and your lifeline. You never know when you'll get a drenching rain that runs down your back and soaks into the pack, or when you'll slip and fall during a dicey stream ford (been there, done that!), so the sleeping bag must be inside a waterproof container. Stuff sacks are not waterproof--stick a towel in one and dump it in the bathtub, and you'll see why--the closure isn't waterproof!

That leaves you with a choice between a dry bag or a pack liner. With a dry bag, test that in the tub before using--not all are "dry"--and at least annually, and check daily for possible holes or worn spots. If you use a plastic pack liner, make sure it's 2 mil plastic (contractor's trash bag or trash compactor bag--if the latter, make sure it's unscented) fastened with a waterproof closure (generally a "candy cane" closure where the neck of the bag is folded back on itself). Also check that liner every day for possible holes or thin spots, and mend with duct tape (replace the liner when you get home).

Of course you'll want to pack your shelter where you can access it and set it up without exposing your insulation to rain. That means your shelter should be on top of your pack (outside the pack liner) or in an outside pocket of your pack. Don't open up the inside of your pack (or pack liner) until the shelter is set up and the pack is brought under cover.