I don't fly without spats
If we had the spats on, I'd take them off. Mud/slush could freeze in flight and lock the wheels.
Other than starting a Lycoming with the primer out if much below zero, I can't think of anything at even the coldest Scottish temperatures. The coldest I've ever heard on Inverness ATIS was -17 when night flying in one of the rare very cold winters. We use Shell multi all year round in the O-200. We rarely have starting problems.
Winterising the pilot in a draughty Jodel 1050 is another story.
I would avoid leaving the plane outside overnight in winter if at all possible.