Your used trailer tires should be always inflated to the maximum pressure recommended by the manufacturer — according to the weight that you are going to carry! You will find this on the official site of your tire’s manufacturer. For example, Goodyear Unisteel G670 for the 245/75R22.5 size has to be 70 psi for 3875 lbs per wheel (highway steer) and 105 psi for 5150 lbs per wheel (highway steer).
Always make sure to check the pressure of each tire on a regular basis! And also — before driving, as it increases with heat.
Underinflated tires become overworked and in time they will get damaged and unsafe (and can even blow up)
Overinflated tires lose traction, the tread will wear much faster, plus they can also overheat and blow up.