Experience and mistily remembered microbiology classes both say that if it's just water you don't need to. I wash my water glass if evaporation has left mineral stains on it and that's it. If you are the kind of person who would think to ask this question you're fine. Neither water nor glass are conducive to bacterial growth.
If you've got a proper water bottle like cyclists or gym bunnies have and it starts to stink (happens more often and faster with plastic, enclosed containers and UV opaque stuff) just leave it in a dilute solution ofg vinegar overnight before washing.
What's the theory behind why the vinegar works? I am one of those "gym bunnies".
With Alicorn's permission, I'm resurrecting this thread.
I'll start off with one of my own: What kinds of exercise can I do at home (I do have 5- and 20-pound weights), and what are good ways to get motivation to do so regularly?