I must be one of the few motorists in the UK who seldom use their heater. I used to, but since buying the Jazz, with its blue low coolant temperature warning light, I have stopped.
I found that by leaving the heater set to Red, the warning light took considerably longer to go out, and in my search for better fuel consumption I turned it to Blue until the light went out. I then found it was a pain remembering to turn it back to Blue when I put car away, and as most of my trips are short I just left it set for cold. Car very quickly warms up with a couple of bodies in it.
When my wife gets in the car for a longer trip, the first thing she does is strips her outdoor clothing off, so I have to use the heater then, but for short journeys or when I am in the car myself I never bother.
I always keep my outdoor clothing on in the car. I have this fear of being trapped in the car after an accident and though being unhurt I freeze uncomfortably, waiting on them to get me out. Irrational, but a possibility.
Just the same as I always empty my bladder before going on a car journey, as I am aware that a full bladder can increase the risk of injury in an accident due to "hydraulic lock" (same as water in a cylinder).