if by top offsyou mean like 10% of the water, I think you are fine, that's what i do now, i used to do like 40% water changes but lately I don't have the time to do it anymore, the only difference i have seen so far is the look of the water it has more particles going around the tank and the fish looks fine.
With only top offs your mineral levels increase. The only thing that evaporates is the water. The minerals and salts remain in the tank. So do contaminants. Their levels will also increase. Conversely the good things in a fresh water change get depleted.
If you top off with R/O water your mineral levels stay the same since R/O is without minerals.
If I didn't EI dose, I wouldn't do water changes unless gh, kh, ph, nitrates, and TDS showed levels that required a water change. Since I do EI dose, I do a 50-60% water change weekly with re mineralized R/O water. I only have shrimp in my tank.
I've been just topping off with RO water for a while now. My pH/ammonia/nitrites/nitrates are all stable. It's heavily planted and not heavily populated 30 gal. I do have a crap load of algae though. Sometimes I scrape that off and do a small water change then.