If your filters aren't causing vignetting (and they shouldn't be if you're just using one), then the reason to use a step up is so that you can share 1 filter on multiple lenses. For my setup, I have UV filters that I use for protection, one for each lens. These stay attached _most_ of the time. I'll take them off if I'm shooting into a light source that's causing flare. I also have a circular polarizer and a neutral density filter in the size of my largest lens, with step up rings so I can attach them to smaller lenses. I'll take the UV's off if I'm going to be using one of the other filters as well.