check the ppm of your water...if it's over 250 ppm, it might be a good idea.
all a reverse osmosis filter does is force water through membranes that have very small holes in them. they filter out objects down to .1 nanometer, which is smaller than most bacteria. they remove almost all impurities from water. they also remove calcium and all other minerals from the water. this means you have to add cal mag to your water each time you water.
tap water is fine for most plants. the small amounts of chlorine and chloramine are actually good for plants at the level they're present. tap water will have calcium, ammonium, and other trace elements. if you remove all of that, you have to re-add it, so the only real reason to get an ro system is if your tap water has very high ppm, because that makes it a lot harder to control whats in your feed
I'd like to add to this, the membranes themselves are bacteriologically dirty. I don't remember the name of the organisms involved, but slime was mentioned as being a cleaning challenge. My unit uses a thin-film membrane, a type which are often installed on wells, and they're available in different gallons per day ratings (get the biggest one, mine is 50, and I wish it were 100). When chlorine contacts the film, it is claimed the film breaks down quickly. The carbon matrix pre-filters remove chlorine and chloramines when the feed is municipal water to lengthen the life of the membrane. The membranes may be cleaned with various chemical baths, sometimes alkaline and acid solutions (must be "pumped" through the membrane, often done on a bench with a recirculating pump for minutes or hours), hydrogen peroxide may also be used, but this cleaning cannot be 100% due to the way the membranes are rolled when constructed. I used to clean them using a different technique, but I don't really think it's needed or that helpful. What seems to work better for me is sanitizing the water post-membrane. When I change the pre-filters (or shift them usually upstream replacing only a clogged one (identifying the clogged one is fun)) I flush the membrane with a capillary-bypass valve installed for the purpose -- pre-filtered feed water is routed through the membrane at a much higher flow rate than normal operation. Some systems use UV sanitation post-membrane, they have a bulb maintenance issue, instead I just boil all my drinking water using stainless steel pots on the stove, actually 6-quart teapots. I don't worry about sanitizing water intended for plants, only animals. I can verify via odor and taste the boiled and cooled water is cleaner, particularly after letting it sit in a glass overnight (tastes great although with the flatish taste of RO water which is lacking minerals). So I am feeding my plants some low doses of bacteria or some microbiological organism, whatever is growing on the membrane, (and which in turn has infected the piping and storage tank).
Here's one cleaning manual PDF titled,
"Assessment of the biofilm removal efficiency of cleaning agents and procedures for RO/NF membranes" (link) There are others if you're interested to look for them. Identifying the type of membrane you have relates to the chemicals you may use to clean it. That's one of the advantages of building your own, you know more about the individual parts.
