1. When uncomfortably close to full, and I'm still going to be there, and the time is convenient (e.g. not busy cooking or doing something else in the RV and the dump station is not busy).
2. When leaving the campground to go into storage for an extended time.
3. When traveling day to day, and tanks less than half full, I'll usually dump on the way in, so that I'm starting camp with empty tanks. If more than half full, I'll dump on the way out, add several gallons of water to the black, so motion can help clean the tank, then I dump that on the way in at the next stop.
I treat full hookups as a conveniently located dump station, and pretty much follow this same schedule.