The pressure marked on the sidewalls is a maximum, usually needed to
reach the maximum load capacity of the tire (which is also marked on
the sidewall) but not necessarily where the best handling is found
with more-normal loads. That, as you surmised, varies from car to car
and is found on a door panel or in the owner's manual or some other
place specific to the car.
See for instance
Racers and advanced sports car enthusiasts might "chalk" tires or use
pyrometers to fine-tune the best tire pressure for what they are
doing, or will bias it in a certain direction to get a specific
cornering result; but regular folks doing ordinary driving, and who
haven't made suspension mods or major changes in the size and type of
tire, will probably find the best advice on the door panel.
If you're going to err, it's better to be a bit overinflated (within
the sidewall rating) than grossly underinflated. But why err when you
can be right? Remember to take the readings "cold" (i.e., before
doing much driving) and to re-check every few weeks or before a long
trip, whichever comes first -- tires lose air over time, and sometimes
just one of them will do it faster.