Block heaters are mainly used only for temps below -18C. It does not hurt to use it though if you can. Every little bit helps extend battery and ENGINE life.
The use of synthetic oil though has all but eliminated my need to use it, except in all but extreme conditions. I will use it if the temp threatens -30C. By then, an engine requires all the help it can get.
Go ahead and use it if you want. I would put it on a timer though to save on electric bill. Only needs to be on for a few hours to do its job. Canadian Tire had one that automatically kicked in when the temp hits -18C. Otherwise it would save your electricity. Not sure how much it costs.
