As others have said, it depends on your riding style.
At Low pressures down to, or even below, the minimum on your tyre you tend to get more grip in slippery conditions but you become more likely to get pinch flats.
At high pressures up to the maximum on your tyre you get lower rolling resistance so less effort to ride, but loose out on grip.
Other issues are:
– Your weight. I’m about 75kg and tend to run with tubes at about 30psi, which just avoids pinch flats. In theory somebody of 150kg would need around 60psi on the same tyres.
– Tyre width. Wider tyres need slightly less air pressure.
– Front pressure can be a few psi lower, to help with steering and because it takes less weight than the rear.
Personally I try to run tyres as soft as I can when it’s slippery – especially important for steep climbs where grip is the limiting factor – and maybe 10psi higher when the ground is firm and dry. You’ll have to experiment with what works best for your local conditions.