Probably only with all-grain beers it would matter to pay attention.
I brewed my last batch with distilled water (close to 0 ions) and added tsp calcium chloride (needed by yeast I think) and some acid malt to lower the mash ph. I was convinced my well water was ruining my beers so I sent it out to have it tested by a lab and it turned out to be very high in bicarbonate 209 ppm. So now I plan on adding one more process to brew day to combat my water situation.
Some links on water that I have found useful lately.
http://www.howtobrew.com/section3/chapter15-4.html
https://sites.google.com/site/brunwater/
http://www.netplaces.com/home-brewing/water-ninety-five-percent-of-your-beer/basic-water-chemistry.htm
http://byo.com/stories/article/indices/18-brewing-science/1525-the-power-of-ph
http://www.ezwatercalculator.com/
