If it's a downpour, I'll check the radar on the cell phone. If there's a clearing for the next day's work, I'll start that instead.
If not, and it looks like it'll clear up shortly, I'll wait in the truck.
If it looks like it's going to be 2-3 hours, I'll head home.
If it's something that I can throw raingear on, and still be reasonably comfortable, I'm cutting.
I only do commercial places. Most of them will never notice if you cut in the rain or not, too much turf to notice.