I would put it on a timer and have it start up the block heater about 3 hours or so before the car's needed. I have been able to start my 83 as low as -10F without the heater but it really beats up the starter. Use the block heater as soon as you feel you need to wear a coat in the morning and give your starter a lot less work in the morning. If it's very very cold where you live, buy one of those jump start things and carry it in the trunk. I've had it save my butt when I had to work too late. Alternately and uncomfortably I go out and take the beast for a drive at lunch time.
Use that block heater, starter's a b***h to change.