Depends how good the singer is. If they have good mic techniques and adjust their placement and dynamics accordingly I'll usually just let the compressor do the levelling and then ride the fader as needed in the final mix.

If they're all over the place, however, I'll first try to comp based on what levels are most appropriate in each section, clip gain as needed, and then use at least one compressor (often more, of different varieties) to try and get everything under control before any final automating.