I think the primary concern is to get the tap water to the proper pH. Use a pH kit to test the water straight out of the tap. Then use either a pH Up or pH Down solution to raise or lower the pH to around the 6.0 range (exact ranges will vary depending on what it is you are trying to grow, but I think 6.0 is a pretty widely accepted "generic range" for most plants). I will actually try to lower mine a bit below 6.0 (more around 5.0 to 5.5 because my pH usually rises a bit after adding the
nutrients). I've heard some folks will also use a chlorine remover much like what you would use in a fish tank. The natural chlorine levels in my water is actually really low, so this isn't something I personally worry about. Not sure it's really needed at all though.