I was down in New Mexico, and it was around the middle of May. It was never a problem until I had to stop and wait for a herd of cows with some bulls mixed in to move off the road and out of the way. Being several miles from town in the late afternoon, I didn't want to get too cocky with those bulls, so I would sit back and honk my horn with the engine running in case they came at me. That's when it tried to boil over a bit. But as soon as I started moving again it was no problem.
I'd say that if you're going to be riding at lower elevations, like Rampart, 717, Carnage, etc. you might think about it, especially if you're going to be doing slow, technical stuff, but if you're going to be doing high speed stuff, or more stuff at altitude like Taylor Park, etc. then it shouldn't be a worry. Just shut off your engine when you stop for a minute, and that helps keep things cool.