dechlor'd tap water is fine for water changes. R/O water would be better for top offs due to evaporation, since only H2O evaporates...leaving all the mineral/hardness behind. If you kept topping off with tap water, you'd eventually have very hard water in the tank.
It should only take 5-10 mins for a dechlorinator to remove chlorine and chloramine from tap water. Prolonged exposure (hours/days) to chlorine/amine will harm fish and plants..but short term causes no problems. many of us with large tanks add fresh tap water directly to the tank, and then add the dechlor to teh tank near the filling point. I've been doing this a while now, and zero problems.