I think I've found the best guide to this. Buy one bottle, refill it until it smells a little off, then chuck it and get another. No need to sterilise, and it'll last at most a month so no risk from stuff leaching out of the plastic.

I got a bunch of "proper" water-bottles for going walking, but they always go manky and the water smells of plastic. I found that reusing springwater/cola/whatever bottles works a damn sight better and costs you nothing. They're a bit less resilient, but they're still well tough enough that you can drop a rucksack on them and they'll stay intact.

Graham.