That's what I've done in the past. I'm sure you're vigilant but make sure you get a good quality timer, with some headroom. My uncle's garage and half his house burned down from a faulty (and properly rated) timer. Took his '67 C10 with it.I'm still waiting for the block heater. Should be here this week. Already got the electric cord. I'm going to use a timer on the outlet for when I want to heat the motor.
Around here it's about 5-6¢/hr to have it plugged in.