Or is it superstition? The thermostat seems to be a simple spring and valve why would the orientation matter? Is it a mechanic habit from thermostats that do matter? In fact 1 brand (waler sp?) doesn't even have an orientation mark.

I've put them in without checking and everything seems fine temp wise. Maybe it will come back to haunt me this summer?! Should I remove mine and spin it around till the mark is facing up?

Any kind of technical explanation of the thermostat mechanism and why it would need to be oriented up is most welcome!!!

(I'm not asking about the direction, that makes perfect sense to me)