Most of our towing is in New England and I keep the cruise control at 64mph when possible. Too much traffic and hills to go much faster. If you aren't in traffic you are going uphill or downhill. Not mountains, but it is rarely ever actually flat.
If I try to cruise at 65mph, I inevitably end up behind that person on the cell phone (doesn't matter if it is "hands free") who is slowing down and then speeding up -- WHY don't people use their cruise control?? For some reason at 64mph I just have easier driving.
I specify where I tow as I keep mostly to New England, so I have rarely been on a road that isn't either going up or down. When we towed down to Florida a couple times I didn't seem to be holding up traffic cruising at my usual 64mph.