First, on every vehicle sold in the US - and I understand this is a worldwide practice as well - there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size. The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.
BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.
One word of caution: Some trucks use tires with the letters "LT" in front of the tire size - and some trucks use tires with the letter "P" in front of the tire size - and some trucks use tires with the letter "C" after the tire size. You should not use "P"'s in place of "LT"'s and vice versa.
For this situation, you should be sure the pressure on the sidewall equals or exceeds that pressure on the placard. If you do this the issue about the letter "C" in the sidewall will take care of itself (which is not true with "LT"'s vs "P"'s.)
If you are using a tire size that is different than what is listed on the placard, then the pressure has to be recalculated. The calculation is not difficult, but it requires tire load tables which are not allowed to be published in the Internet because of copyright laws. However, I have a copy and would be glad to do the calculation, but the starting point is the vehicle placard: Tire size and inflation pressure. I will also need to know the new tire size.
Second, on the sidewall of every tire will be information about loading and pressure. It will take one of 2 forms;
Max Load XXXX, Max Pressure YY
- OR -
Max Load XXXX at YY pressure.
Note that the first one tells you the max for each, but doesn't tell you the relationship, where the second one tells you they are related but doesn't tell you about the maximum pressure.
The reason there are 2 different forms is that the regulation is worded a bit peculiarly and it is unclear which way they want it. So be careful when you read the sidewall. Make sure you know which way it is stated.
Now to the original question:
Personally, I like a car that handles crisply. So I use 3 to 5 psi above the placard pressure. I also get better fuel economy, better wet traction, better snow traction, better tire durability, better tire wear, but I do have a degradation in ride harshness (but I am willing to put up with it.)
I don't recommend anyone use more than +5 psi (with a couple of exceptions)