I use RO water, my TDS meter reads 0 so that's good enough for me.
Tap water is somewhere between 5-10.
I have noticed in my earlier days of reefing that when I used to top up my tank with a cup of tank water I almost immediately noticed a bloom of hair/film algae and diatoms.
so since then I've invested in a filter.

It also has to do with what the water goes through to get to you.
If you live in an old area with old pipes no matter if the water coming in is crystal it's going to leach into the water. so those who use tap water and have no problems are extremely lucky.

In Vancouver we are said to have the best water in the world, and I believe it too. however in a recent news story it showed how despite city testing at the plant, some areas with old/poor piping had severe water problems including smell and discoloration.

I guess what I'm trying to say is RO eliminates any doubt and inconsistency in water parameters.

the large reef tank at Big Al's near the High Tech Section is all Tap water it looks amazing, myself I use tap water, no problems other than a few small cyno outbreaks and in my new tank a big diatom bloom nothing that wont be cleared up with a little TLC.