Don't lug the engine.
Basically, you have a slow moving piston but a fast and power explosion. That force of combustion has to go somewhere. You want it going to the wheels, but lugging results in combustion that can't effectively push down the piston and you just waste fuel and wear the engine.
Besides, the optimal operating range for our engine is somewhere in the 2,500~3,000 range, right? It sure seems that ways to me. If you're doing anything other than coasting on flat ground, I'd downshift and keep it over 2,000 until you want to coast again, then go to the highest gear.