As you can adjust pressures all the way up to race start I would suggest changing them to suit each run in qualifying to be as fast as possible. If softs overheat lower the pressures till they dont, remembering you only have to worry about them overheating over a single lap. So for tracks with long straights I find slighter higher pressures give greater top end speed. I raise them till the tyre is getting a little warm by end of lap.
Lowest pressure setting in the mod mean much greater tyre wear (more than base game) so this is why you will see AI running longer on tyre than some players. They tend to use default pressures.
You can use this by running higher pressure which has less tyre wear (again less than base game) allowing much longer runs
Then at start of race I adjust the pressures for better longer runs and depending on what strategy. If planning to run hards on Monza for example I definitely want higher pressures to stop them cooling on the straight. If this means the softs overheating then I do an early first stop to get off them. Saying this I am definitely not the fastest racer out there and have no where near mastered it and still learning every race. Everyone will be different.
But it is definitely not the case that lowest pressures for every track is the fastest. I find its also the same for brake bias and differential. Differential so important in wet weather. You can stop the back end twitching and get power down faster. I am changing both on every track and even as the race goes on.
Sorry for the long reply