Hey man, I do the same thing but i haven't ever tested the hardness (no ec meter) or any mineral breakdowns or anything. All i know is no chlorine and its ph balanced. I'd love to hear a real answer to this too though
-be easy
Dr.J
Most of the self serve machines are reverse osmosis units, so yes that water will work very well for growing. It may not necessarily be safer but if you have really hard water or salt softened water then it is probably a better option for you. Just a word of caution. Reverse osmosis water has a very low ppm and has almost no calcium or magnesium, which cannabis uses a lot of both of those elements, so you may need to consider some sort of cal/mag supplement. Happy growing my friend!