If the fish just got over an illness, if there is a real problem with dirtiness (which doesnt happen since I feed 1 pellet at a time), and every time I need to transport them from PA to NY, there is a 100% water change. However, I otherwise just do the 25-50% and have never had a problem.
I should add that every time I do a 100% water change, I boil the silk plants and gravel to kill algae and any harmful stuffs in the tank. I realize that there is beneficial bacteria in tanks, but I've never had an ammonia problem, and since all of my fishies are rescues with chronic health problems, I'd rather be killing parasites, ich remenants, etc. and risk the good bacteria. So far its worked like a charm.