I didn't use a block heater for my first tractor, and even at 30 below it always started. Made a heck of a lot of protesting sounds, but it started.
My second tractor came with a block heater installed for free. The first winter I never plugged it in, and the tractor started just like the old one -- plenty of protest but it ran. Then one night I plugged in the heater. Next morning, bitter cold, wicked nasty weather, I started it up. Absolutely no sounds of protest, no hesitation, and a much faster warmup. Been using the heater ever since.
It might not be necessary, but my old girl deserves the best!
Pete