Turning on the heater can sometimes help, sometimes hurt depending on the
car and the temp outside.  The reason I say this is that the heater core is
quit small and if not enough air is pulled through it it won't cool the
coolant passing through it as much as if it went through the radiator.  If
this is the case then the engine doesn't get a benefit from doing this.
I've noticed in my van that the temp gauge will go up slightly when I run
the heater and the blower is not on max.

Dave K.