I would get a waterproof bivy/sleeping bag cover instead of treating the bag. That way when you use it under normal conditions/nice weather, the bag breathes to let your perspiration out rather than collecting it, and in wet weather you can tuck the bag into the waterproof cover and dry it out fully later.

People get paranoid about down getting wet. If you're somewhere humid or rain breaks out and you haven't somehow protected the contents of your pack, I can understand worry. In a campout in the Sierra (for example) where weather is unpredictable but not monsoon-like, keeping the bag in a dry sack and using a pack liner or cover then only unpacking the bag inside the dry shelter out of the rain shouldn't be difficult.