Is There a Reason Not to Do This?

If you mean something like NikWax to rejuvenate the DWR then that is a great idea. If you are trying to make the fabric completely waterproof, that is probably not such a good idea because the breathability would be compromised and you could get condensation inside the down fill.

I'm not trying to make it "waterproof" — I'd just like to not have to worry if the head or feet brush up against a damp tent wall.

I can't refresh the DWR, because the women's Helium doesn't come with DWR in the first place…I was just thinking about spraying on some DWR spray, like Sno Seal Perm Water-guard spray.

Yes, your Helium does come with a DWR treatment.

"Main Material:
20 Denier 100% Nylon Mini Ripstop DWR 1.1 oz/yd"

Yes, you can add DWR to a fabric that does not have DWR. I use NikWax TX Direct to ADD DWR.

OK, thanks — so it should be no problem to spray the head/foot, just to set my mind at ease…

FWIW: I've always been very impressed by how well my Hydrogen repels wetness and condensation; I've never rejuvenated it either. This was proved to me once again just about a month ago in very wet conditions. I have a Helium and can only assume that the shell fabric on this bag has excellent water repellent qualities as well. (I haven't been in conditions to test this with the Helium.)

Actually, the high-end Marmot bags come with durable water-repellent (DWR) outer shells. They used to use Pertex; I don't know what they use now, but I'm sure it's similar. That is a DWR fabric. (I just checked online; the women's Helium still uses Pertex.)

Take a spray bottle with water and mist a spot on the outer shell of your sleeping bag. (This is a test I always do when buying a new sleeping bag, as soon as I get it home) The water should bead up and not soak in. If it does soak in, treat the entire outer shell with one of the DWR sprays made specifically for down sleeping bags. This is important–don't use a general purpose spray which could mess up the down or not be breathable enough. I use ReviveX Spray On Water Repellent. I suggest following the recommendations for care on the Western Mountaineering website. You'll need to retreat the outer shell each time you wash the bag. Be sure to put the bag in the dryer (lowest possible heat) after the treatment to activate the DWR. Actually, if your bag doesn't pass the spray bottle test, put it in the dryer for about 15 minutes and then try again. Often the dryer heat is enough to boost the DWR.

You definitely do not want a waterproof coating or even a waterproof-breathable (WPB) shell because your body moisture will condense inside the bag, especially when it's near freezing.

Valerie, I use the "wash in" NikWax TX DWR vs the spray. I can't comment on the spray, but the wash in works extremely well on Pertex fabrics.

Someone else can chime in with respect to spray.