I rarely wash the insides of my bottles. I only carry and drink water, and the only exception is occasionally on a very long ride I'll stop at a convenience store and put some Gatorade in my bottle. I make sure to wash the bottle when I get home, but other than that, I'll leave a half empty bottle of water in my garage until the next time I ride. The outsides of the bottle get rinsed off after gravel rides, and I do occasionally clean the mouthpiece more thoroughly.
I make it a point to wash my coffee cup at work at least every week. But I am a cleanliness freak in other areas.