You can back off after a few weeks into flower, or you can use it clean up 'til harvest time. I've done it both ways and couldn't tell any difference. I never flushed, either. Tried with a control group. No difference. YMMV.
Well, I use one. Tap water is ph 8, sucks for my plants. Was using distiller, 3 hours per gallon. Went with RO, takes about 12 minutes a gallon,
6.3 ph. That's why I use one. Sheesh, I won't even drink tap water, nor feed it to my pets. Not until RO filtered. If you ever smelled the left over in the Water wise distillers boiler, you wouldn't drink it either.
I use an r/o filter and I pay to put cal/ mag and other supplements back in like Hippie said. But like Bubba says I don't trust what is being added to our water. I won't drink unfiltered water....why would I smoke it.