As a practical matter, some mtb rims aren't rated to road bike pressures, although some of them allow you to use higher pressures with smaller than mtb tire sizes. That always seemed questionable to me. Some stan's rims are max 37psi.
Of course, some manufacturers never bothered to spec max pressure, which makes me think that sticking with mtb pressures is a good idea.
I like low pressure on rough surfaces. I often go a little too low and notice my steering is scary taking a hard corner on the road. Off-road, the same pressure feels fine during cornering. The rolling resistance sites will let you do a comparison. They often play with the y axis to make the rolling resistance as a function of pressure look more pronounced. It's generally not that much. Nobody I know of has figured out how to do a comfort comparison