the heatercore is part of the thermostat-shut circulation it actually warms your engine up a lot more evenly and it doesnt matter if you dont have a shutoff valve on it, it circulates all the time. you have a flapper door in your hvac box that closes off the heatercore airflow and the air goes a different route. if you put a shutoff valve on there and shut it down, i dont imagine it causing problems. heating the coolant in the block first then dumping cold coolant in there might cause some wackyness for a min or two but nothing to get flustered about. so run the heat whenever you want.