I've used distilled water for wheatgrass with good results.
One thing I do that you might want to look into is prill beads. These are energized magnesium oxide pebbles. They both pull out toxins from water and alkalize it. I transfer distilled water into a glass jar with prill beads in it, let it sit for a couple hours and use it for both myself and plants. Wheatgrass seems to like it. I also have a herkheimer crystal in the water. One of these days I'm going to get serious about testing water elixirs with various crystals. Anything to make the water better.