I've been using tap water - it never even occured to me to sterilze it, and my doctor never told me to do so. However, I just asked my husband, who has a PhD in water disinfection about this, and here is what he said (I copied and pasted Christy's response in an email to him):
"The potential exists...."so much" is misleading, but bacteria do grow in tap water, If you measure tap water for HPC (heterotrophic plate counts), it grow some - you may see 0-500 cfu/mL (colony forming units). I think the EPA wants less than 500 cfu/mL at all times. I think typically you see 0-50. Pseudomonas aeruginosa is an HPC bacteria that probably lives in Austin. However, you are adding a lot of salt to the water, which should kill some of the bugs off. However, other bugs, like the ones that cause pneumonia may also be present in tap water (HPC). So........I feel I should of thought of this.........you should probably sterilize the water......I feel an experiment or some research could be done on this issue as well."
We are going to be buying one of those UV water treatment things REI sells this week.