Rule of thumb in brewing is always: if the water tastes good, the beer will taste good.
For extract brews, it probably dun matter that much. For mashing though, the ions content will make damn big a difference.
I am not so concerned abt microbes since boiling will get rid of them significantly. I am more interested in knowing if the water has any kind of residue, etc. It will be prudent to filter the water first I guess.