I'd second those saying to try a few different pressures yourself and see what the difference feels like.

Normally I'm a "don't care too much about the pressure, pinch and make sure it's not flat" guy, but my last trip, I found when loaded down the tires had to stay near the max or it was like dragging a boat anchor. Wasn't an issue unloaded, but max pressure on the tires were 80, when it dropped into the 65-70 range it was a very noticeable issue.