The thing you will find about water on RIU is that people seem to forget we don't all have the same tap water. My opinion is that if we filter the water, we get most tap to a pretty close starting point, PH aside. like I said, I filter and theones that attach to the faucet, as opposed to normal brita pitcher, filter out more (chloramine in particular) plus they are easier to use.
I let my water sit out a few days. I have about seven one gal containers and as I use one I fill one, keeping the rotation in order.
I water via a halo drip system out of a 5 gal bucket. I do run an airstone in the bucket for about half hour before I turn on the spigot (check out my journal for vid on set up, I love it). I don't know that the airstone helps, but my plants are happy so why fuck with it.
While many will say othwerwise, my problems all disappeared when I became religious about PH. Some people don't need to, soil does buffer, lime does help, but some of us still need to PH.
Still, buying water is costly and a hassle. Prob not nevessary.
RO, I think, is more important in hydro. It is the only thing that gives you a truly blank canvas to start with. But my advice would be to buy a filter, let it sit for a day, And PH. Do that and you will be fine.
I water every third day because I don't trust my judgement. I use 3 gal pots with a mix of soil, peat moss, vermiculite and perlite. I water until I get a little bit of run off. On my last grow (4 plants), every third day worked perfect with the exception of one incident where the two plants closest to my fan got a bit too dried. But once I watered they perked right back up.
Good luck!