I use a Pur filter that attaches to the sink. While I am not a subject matter expert, water only has an effect on the mash and that is all pre-boil. I can not say I have an issue with "topping off" as I boil to my target volume normally 13.5 gallons to 12-12.5 gallons. Using exotic water seems a bit silly to me esspecially for extract brewers.
Do you know how to make crystal clear ice? Do you know what causes haze inside an ice cube? Let me explain and feel free to try this at home!
Take a pot of water and boil it. Take it off of heat and let it cool to room temps naturally (artificially may also work never tried it that way). Carefully decant the water like you are pouring a homebrew leaving the bottom water in the pot into a different pot. Boil and let cool as before, decant and freeze it. Those ice cubes will be crystal clear!
So what was taken out? well, minerals and oxygen mostly but other contaminants as well. While we do not double boil I contend that minerals "fall out" during the boil anyways...not all of them but a good portion.
Use what you are comfortable using. I started out using Ice Mountain and have now switched to tap. The only place water matters IMO is the mash and then it is a matter of the right PH and minerals per style, if you care about it that much. (some folks do.) The best rule of thumb I follow is pre boil, no real worries, post boil anything that touches my wort gets sanitized at the very least. My water filter has nothing to do with anything lol.
I have only had 1 infection and it was 100% my doing. Meh, it is a sour beer now...lol.