Unless it's undrinkable, I would have no problems with giving tap water a try. The only exception, is if the muni water co. adds a solid form of chlorine to kill bacteria. If so, R/O would be necessary, as the chlorine solids will kill the beneficial bacteria in your soil. But I'm not at all a fan of buying water just because someone else is paranoid about tap water. Usually, those in soil, that are so anal about water that they have to buy it, are those that think they need to have every garden ammendment, fertilizer and tea to properly grow cannabis. (Advanced Nutrient and Lucas Formula nazi's for instance) That's just not the case.

Perhaps sometime when you can plan for potential failure, (doubtful but possible) try one plant from start to finish with (properly ph'd) tapwater. See what happens. Worse case...you revert back to paying more for watering for your plants, than you do for gasing-up your car. But other than your wallet thanking you, your back will thank you for not hefting water all over the town, and your complection will improve. (threw that last one in there to see if you were paying attention)