Should I use the heater while warming up my car?

If I understand things correctly...the thermostat will not open until your car reaches operating temperature. So at first, you're just blowing air over a cold heater core. As long as that thermostat remains closed, and coolant isn't circulating, I suspect running the heater neither affects, nor is affected by, your engine temperature. Once that thermostat opens, though, and warmed coolant begins circulating, that's when you should have an exchange across the heater core, and that is the point where the heater being enabled becomes relevant..

In other words if you leave your car running in the driveway to get it warm, it should begin to heat up just as fast with or without the heat going...you don't need to make a second trip out to the car just to enable the heater. While it might reach the warmest temperature somewhat slower with your heater running from the start, I think the difference will be negligible. (You could always test it.. :)

As to your other observation, yes you can enable your heater in the summer if you are experiencing overheating, and it will provide a secondary/auxiliary heat dissipation surface. But you shouldn't rely on it. I only do this when I see my temperature begin to rise above normal. I'll enable my heater, and I'll give it about 30 seconds to a minute to make a positive effect. Thirty seconds to a minute, tops! Then, if the temperature continues to rise with the heater enabled, I pull over before I truly overheat! By the way...if applying the heater in this situation works and your temperature decreases, it signals an immediate need for some cooling system maintenance. In my case it's usually been a low coolant level, or possibly needing to "burp" the radiator.

Final thought. Contrary to your first statement:

I know that you should drive off immediately to warm up your car..

I believe it is better for your engine to allow it to reach operating temperature under idle conditions, as opposed to reaching operating temperature under load (driving conditions). When you start driving away cold, you're effectively putting very conditional, yet very avoidable, wear on several moving parts. If it were an airplane engine, taking off without a warm engine would be considered downright negligent...I'm just telling you that to underscore the performance relevance of a cold vs. a warmed engine. Obviously, the consequences of a cold automobile engine failing, or not performing as expected, are very different than those of an airplane.

Well, this is a question for a Canadian guy. Here's the thing:

• in extreme cold (-20C and colder), the moisture coming off your breath will condense as ice and fog on the inside of your windows. Air circulation, whether cold or hot, will help with the fog part
• your engine does not need to be at operating temperature to defrost your windows, even just a little bit of heat from when the needle starts to move is enough to defrost your windows, it will just take more time
• unless I'm mistaken, the thermostat does not block off coolant circulation to your heater core, it only blocks it to the radiator.

First, find out yourself what the real temperature of your engine is and how fast is it warming up while you’re there idling. Keep in mind that dashboard gauge is pretty useless, inaccurate and often rounds up the temperature data. Google if your car has the ability to see your coolant temperature digitally.

For example, in my audi 2001 1.9tdi, there’s key configurations I can do to see digital accurate engine coolant temperature.

One guy here said blowing warm air to heat up interior will affect your engine temperature just a ‘tiny bit’

WRONG

Blowing hot air inside dramatically reduces engine’s ability to warm itself up.

For example, when there’s -10C degrees outside, and I’m blowing heat inside interior, my engine wont heat up to operating temperature in the city no matter how long I drive. However, if I turn off the heating to the interior, my engine gets to operating temperature in like 15 minutes of driving.

Also, if it’s really cold outside consider inserting a large piece of cardboard before your engine to block off incoming cold air. (This in my experience helped a lot to heat up my engine quicker but be sure if doing this to not block the whole air intake and you must be able to see accurate engine’s coolant temperature so as to not overheat)

TL;DR Don’t blow heat inside your interior if you want to heat up your engine quickly because all the heat you get inside is heat stolen from the engine itself.

I have always been a proponent of not running the heater until the engine has had a chance to warm up...as a large part of the wear and tear on an engine happens when it is cold. However, after reading the comment about the thermostat remaining closed until the engine warms regardless of whether you turn on the heater or not makes a lot of sense. The thermostat will block water flow from the block to the heater core until the water temp reaches operating temp (180) before it allows water flow to the heater core. Preventing the heater fan from removing heat from the engine. My only concern is if the oil in the engine IS NOT at operating temperature when the thermostat opens, if that is the case, running the heater with a cold engine will prevent the oil from reaching operating temperature as early as possible. And the proper operating temperature of the oil is most important.