Long story short, if you're growing in a very high output system (aero, DWC, NFT, etc.) without a medium than an RO filter is almost assuredly a good buy.
If you're growing in a slower, more forgiving medium like soil (or its cousin hempy buckets), than you can get away with not using RO because of the buffering capabilities of the media.
The above two statements are rules of thumb and not set in stone - if your tap water is 50PPMs than you can do whatever you want, and if your tap water is 900PPMs I wouldn't even water my lawn with that.
I'd recommend anyone who plans on growing in hydro to buy a pH pen and/or TDS meter and test their water to determine if they need an RO filter.
All the useless trolling of this thread is kinda silly, especially by the guy who started the thread (IMHO).