See, I always heat my bathroom, because it may be little time I spend there, but I loathe feeling cold while I use the toilet or come out of the shower. Also my bathroom doesn't have a window for airing well so I need to heat it or I'd have serious problems with the humidity encouraging mold. On the bright side with the relatively little squaremeters it has, heating it doesn't amount to much money. (The system my place uses is that some base cost is not dependent on usage, then the rest is determined by what the meter on the radiator shows multiplied with a factor for the room size.)

I don't heat my kitchen (unless I have guests), because it is a large room, which is nice for sitting there, because many people can sit there if I unfold the kitchen table fully and it is nice for cooking too, but when it is just me, it's just not worth it. Thankfully the recent renovation and repainting of the building's front took care of one of my kitchen corners being persistently clammy, so I had a spot of mold there recurring every winter. Which of course would have stopped had I fully heated the kitchen regardless of the state of the outside, so I could not complain (as per the lease obviously I'm required to heat and air sufficiently, but I was miserly). But it's not been a problem anymore with the new paint repelling the outside humidity better and I can take care of the cooking humidity by just airing.