new rebuild startup and thermostat?

I always remove the stat for cam break-in, and replace it with a restrictor washer. You got enough things to watch over at this time, never mind worrying that the Stat will open on time.
I always run straight water at this time too. #1, it is a better coolant, and #2, if some pukes out on the floor, it is easy to clean up, and cheap to replace.