I've never had an issue topping off... As long as your doing 25% WCs on a regular basis and your evap isn't to crazy. and actually topping off my SW tank w/ no WC's for a few monthes imo was what induced my pipefish and perculas to breed.. however i don't have any hard evidence to support my theory on that one.. So I don't reccomend it.
in general applications with hardy/ non-sensitive species ect. declored tap imo is fine. and depending what species your keeping. ie African cichlids for example.. useing R/o , Distilled, ect may induce PH dips in your tank, and stress your fish. or if you keep low PH loving species they may enjoy a few ticks down on the PH.
My view on it, If it isn't broke don't fix it. If you have been topping it off, and not had any issues. I'de continue doing it. If you have issues w/ fish being healthy and have exhausted all other possable options.. well.. there ya go.