Hello from the frozen North (Canada)
I have installed lots of block heaters in the Volvo red block engines. They are placed in the rear welsh plug on the exhaust side. The factory plug is removed, and the rubber and aluminum plates seal a heating coil, just like the one in your kettle, in the block.
There is no need to avoid the use of them at any temperature. In moderate temps they just establish a higher temp in the engine. Many fire truck engines use them in order to have easy starts and full power available as soon as they drive. Also the interior heater will work far faster.
They cost about $45.00 here, and are easy to install. The kits are specific to the engine type however, so buy the right one for the engine -don't try to swap the diesel one.
Almost all GM cars sold in Canada have them installed at the factory. My 94 Camaro z28 had one ready to go.