With the kind of use boat trailers get, it is pretty rare to wear out tyres, they usually perish with age. Personally I wouldn't bother rotating them.
As for tyre pressures, the table shows the maximum pressures and carrying capacity. If the carrying capacity of your tyres is way over what they are actually carrying and the trailer skips around a bit when hitting bumps, you may want to reduce them a bit. The object is for them to have enough give to not rattle your boat to bits but enough pressure so they don't get too hot at speed and cause a failure.
Hopefully someone more knowledgeable than I can chime in with something less vague in relation to the tyre pressures you should run but if the trailer feels good at 40 and your tyres can take 40 (check sidewall, should be written there) then stick to 40.
