Depends on the plant, the medium, and the water. Get a PH "kit" either way, you'll need it. As for whether or not you'll need to PH your feed will depend on the medium you're using and that strain you're growing. My setup I'm learning that not PH'ing means perfect results with feed. This is because my water just happens to match my mediums characteristics. I'm using a soilless mixture that tends to dip into the low regions of 5 PH. As I need to keep the medium around 5.8-6.5 range I can water safely with my tap water, left out for minimum 24hrs to acclimate/evaporate, with a PH of 7.5. Before anybody bites my head off, feeding at less has caused irreversible damage to my current grow. This is comparing to the same setup, same strain grown last time.
To your point - use RO water if anything, don't use distilled. Adjusting PH will have to be tested, start at your safe point though