Depends on the vehicle for me. In my diesel, I used it pretty much everywhere, mountains, flats, whatever. The vehicle had no trouble keeping the speed right where I wanted it.
My 1/2 ton Suburbans have had a harder time keeping the speed right there, and will lose speed on a minor incline, overpass, etc. With the cruise on, the truck falls all over itself to keep it right at speed, and I didn't like the redline shifts to pick up the 4-5 mph I lost when I knew that I'd be able to make the speed back up on the other side of the hill with constant pressure on the pedal. So I almost never use cruse towing with my Suburban.
I haven't towed enough w/ my F150 Ecoboost to know what will work yet. I expect it will do much better than my Suburban on flat or small inclines, but I will find out soon!