I don't bother. I've used them for years and when back from a trip, I simply leave the cap off (or open, pull tops). Water is the only thing I ever use them for and simply rinsing them out before next use is all I ever do. Never had one go bad or grow critters inside.
Water is inert...it's whats in the water that mattered. Just rinse them out before re-filling and you'll be fine.