Remember: Towing a trailer, you aren't driving your rig, you are just AIMING it!
Just depends on how wide the yellow stripe is down your back. Mine's pretty wide, so I hold it around 60 to 65. You will get better gas mileage too. About all those people passing you, that's their problem. If they rear end you, the trailer will take up most of the shock, and it's their fault.
A friend of mine felt what he thought was a road bump, stopped to check, and found a Harley with rider embedded in the back of his Airstream. Unfortunately, the Harley rider didn't survive it.