Working underneath a trailer--best way to do it?

If it's really a concern for you, just pick one out that's already high enough. Or get an "axle flip" done after the fact (if your new unit came with leaf springs). The only advantage that I can think of to the lower trailers is the amount of effort required to get in and out of them. Some people say they get better towing mileage and some people say they tow more stable. I don't buy into either of those arguments, and prefer good ground clearance.