I am using mountain spring water from the big jugs. If I open one of those fresh to add to the wort to achieve the appropriate batch size after the boil, Should I still boil it to sterilize it?
Things that cause infections arent usually in tap water due to the chemicals most water municipalities use. Run your water through a Brita or PUR filter and your tap water "should" be good enough to use. Filtered "spring" water is usually no different since they run it through all sorts of filters.