Gary's graphic says it all, but I think a few words of explanation will be helpful.

The spherical mould filter works by stretching a bitmap around a sphere. Visually, the centre is enlarged by 57% (i.e. multiplied by 1.57) and the edges are compressed to (almost) 0. Any jaggies in the central region will be magnified by up to 57% and may look even worse because of the circular distortion. Continuous tone photographs will be less affected but text and graphics should probably be oversampled by a minimum of 57% to maintain quality in the central region. Graphical images which have not been anti-aliased may need greater oversampling because of their inherent 'jagginess'.

Note that if you're going to increase the dpi of the bitmap by 57% or more on the page (i.e. by reducing its size or increasing the dpi of a fill), there's no need to oversample the bitmap.

Regards - Sean