That would depend on the quality of your tap water. I have well water, so that suits my snakes just fine. But the "city" water in my area isn't fit to bathe in, IMO. It contains so many chemicals and stuff to make it fit to drink, that it isn't fit to drink, if you know what I mean. And all well water isn't the same. I grew up in a house that had the bad luck of having a sulfur water well. I didn't know what "real" water tasted like until I was in college.
As a general rule, if you feel safe drinking your tap water, it should be fine for your snakes.