I think this is a case of straining the knat and swallowing the camel. If you boil the bottled water you can be sure that it has no bacteria in it, reducing the likelihood of an infection. However, you are also increasing the cool down time of your wort. The longer the wart sits without yeast, the more likely some bacteria may start growing in it first. I don't think it's a worthwhile trade. I top off with plain old tap water (filtered) and have never had a problem with an infected batch.