I live in south-coastal British Columbia. In the depths of winter the average temperatures here are 2°C overnight low and 7°C daytime high. Of course those are averages, we do get the occasional cold snap, and the occasional warmer day.
I've read that Winter Mode's battery warming can kick in as low as 15°C, but it seems like its function is most important in sub-zero temperatures. That makes me think that it kicks in at 15°C to get ahead of the really cold temperatures, rather than it being necessary at temperatures that high.
Charging is done entirely by a 120v outlet. Range is not an issue either, the car is regularly getting about 450 km on a charge since very little driving is highway, so I'm not concerned about the range hit. I am concerned about unnecessary wear and tear on the battery warming systems, though, since the plan is for this car to be used for decades and we want to baby it where we reasonably and conveniently can.
Given the available information, would you recommend Winter Mode never be turned on? Or be turned on only during cold snaps? Or be turned on for the entire winter?