They are both absolutely right. If your plants do well on tap water.....by all means continue to use it. When I looked into RO systems I was told (by someone with very good knowledge) that "regular" water is very good for your plants....IF....as the guys above me state.....you can control the ph. With my well water I couldn't control the ph which was why I went to RO water.

PC's link to the ph discussion is very good and goes very much into detail about different types of water and ph issues. i'd post a photo but you really wouldn't see anything....the water looks the same both before and after.
The numbers go pretty much like this....
Well water........ppm 150-180, ph 6.5-8ish
after RO...........ppm 10-15 ph 6.2-6.7 (ish)

I don't do anything else to my water before I use it.... besides adjusting ph and making sure the temp is good for my plants and fish.