The problem is, then, you have to know your minerals. Let's say that your substrate is full of magnesium but not calcium. Distilled water will then leach magnesium out of your substrate and calcium out of your frog.
Fact is, distilled water will leach various minerals out of various things. You just don't want one of those things to be your frog. So the problem is sort of complex.
Over the long run, it's best to use water that's already got a fairly balanced and typical mineral content.