It also depends on what your incoming water is like. For locations with water that has saturated CO2 or little buffering capacity, you'll get quite a bit of fluctuation in the tank when you do larger volume water changes. Here, I regularly do 80% changes on bristlenose growout tanks with no problem. I've lived places where 40% changes stressed the fish though. It really just depends on your individual conditions and parameters. Watch the fish. They'll tell you if you're doing too much.