Totally dependent on... Everything. The mic, the vocalist, the song, the arrangement, positioning, lead/backing vox, the room...
The best starting point is "ON."
Get someone to make some noise while you fiddle with the controls - After thoroughly reading the manual, of course.
And keep in mind that just because something is available (compression, limiting, etc.), it doesn't neccessarily need to be used all the time - Or at all.
Listen, and do what the source is asking for.