Step-up ring or get the right size filter?

I have the GF1 and the 14-45mm and 45-200mm zooms. Part of the appeal of these lenses is that I can use my many Nikon 52mm filters. I want to get the 20mm lens next. Should I buy a step-up so I can continue to use my 52mm filters or buy a 46mm filter? Ant reason not to use a step-up ring?