It will not hurt to leave the valve open until you can get the servo hooked back up. Usually, when the heat goes cool at idle, it is because some air remains trapped in the heater core. Another thing that might cause the issue is lower pressure on the system due to a bad reservoir/radiator cap.

 

it is fine to leave the heater control valve open in the winter, but once summer comes and you need A/C, this valve being open with pretty much kill any semblance of cooling in the car, so you have a few months depending on your climate.