RO/DI TDS creep...

When the water is shut off on the RO, some of the tap water impurities are pushed into the clean water side of the membrane.  Let's say there's 1/2 cup of "tainted" water sitting in the filter.  Now lets say you're using a float valve (or single float switch) for auto top off and it cycles on every 30 minutes and delivers a cup of water before shutting off.  Now every 30 minutes you're getting 1/2 cup of "tainted" water and 1/2 cup of clean RO water.  So in essence, half your make-up water is tap water.

Counter that with a system using two float switches that do nothing as long as the water level is between them (mine is about 2" of deadband).  When the level drops down to the lower switch, the RO is turned on and the sump fills until the level reaches the upper float switch, then it shuts off.  Using this method the switches only cycle every 8 hrs (but dump 16 cups of water each time).  Now I still get the 1/2 cup of tainted water at the beginning of each cycle, but overall only 1.5/48 = 3% of my make-up water is tap water.  Definitely lessens the impact.  And reduces the duty cycles on my solenoid valve.

Back flushing each time you use the filter is even better (but imo, overkill).