My new truck has a block heater, but all my previous ones never did. A good heavy horse blanket over the front and an electric heater always kept the truck thawed out even down under 20*, unless there's a hard wind. It really doesn't take much to keep the block from freezing, but you definitely will do some sort of damage if it is left out unprotected in freezing temps for extended periods.
When I was living in Central BC where -40* temps were common, I used to leave a 1000w heater under the hood and a 500w heater in the cab. Sure was nice to get into at least a half-warm vehicle in the morning! Just gotta make sure to check well for cats sleeping in there in the morning though, they sure loved it too. LR
'09 Ford Ranger FX4 that gets abused a lot... but doesn't like to swim though.