I've found "sweet spots" where my rig cruises nicely at 62, 64 & 67 mph (based on my GPS readout), so that's where I tend to run it down the interstate. When not pulling the toad, I've hit 75+ a couple of times for short bursts, but I certainly don't typically cruise at those speeds for long periods.
On secondary highways, I'll keep at the posted speeds and on two lane roads I'll typically find myself below posted speeds to reduce constant slowing on curves, blind intersections, etc. I'm always trying to expect the unexpected.
If, when on a two lane secondary road, I notice traffic starting to line up behind me and the road is not condusive to increasing speed safely, I'll try to move over and let traffic by, provided that there is a safe place to do so.