? much help will 3/4 ton make for ride quality comp to 1/2?

Before I went to the 350 I was looking at new tires. The place I we t to told me that if I went from P to a LT tire I still had to keep the tire pressure at what the door said and not to pump up to what the sidewall said. Did not make sense to put tires that could go to 44 or 65 or 80 on the truck but only run them at 35psi.