The use of step-down rings is not for mounting filters, but for other accessories. For example, many Coolpix users mount two tele converters together to create a converter of higher power.
However, when using an SLR lens made for a film camera on a Digital
APS-C Camera. You normally can go a size or two smaller without vignetting. For example, I have a Sigma 50-500mm lens with a filter
size of 86mm and I am using a step-down ring with a 77mm 81B
Hoya filter and I have no vignetting problems on a APS-C Nikon