I usually primer black for a fast tabletop job; white if I want to really make a top knoch figure (top knoch for my standard, of course).
Browsing in CMON I found an awsome technique: underpainting with zenithal light source. You prime black, than cover all unpainted areas with waterd down balck, than, with a white primer, you spray from above, as if the sun was hitting the mini from the zenith.... It works wonders.
cheers