there is a side of seasonic 1250xm that gets hot like when you touch it you would remove your hand immediately, that side I would not dare lay it flat on a surface...other PSU are of different make and build but still..you get the idea.
The side away from the fan - but if you have it on a metal shelf it's a GOOD idea to lay it flat, as the shelf will help dissapate the heat some.
Wood or any other insulator, flat would be bad.
The way my setup works, all the PS are on their sides though for space and air management reasons so it's not an issue either way.