Risk vs. gain: how much economy would you gain by leaning at 2000ft - half a gallon per hour? Compared to the increased possibility of engine damage if the power setting is slightly above 75% I believe the safest way is to fly on full rich on cruise density altitudes lower than 3000-3500 ft.
You definitely burn some more Avgas that way, but at least you know for a fact the engine isn't over lean...
Perhaps you misunderstood me, I didn't say leaning is bad, but you shouldn't lean automatically when you reach cruising flight level, you should take into effect other factors, such as: length of cruise (for example, you probably don't lean in downwind when doing traffic patterns?), ability to maintain altitude (I don't think it is very wise to fly very lean [peak or close to it] when you're unable to maintain +100/-50ft due to turbulence), ... Some instructors teach to fly full rich all the time, some teach to always lean when in cruise (regardless of circumstances) - the right and the safest way to do it is probably somewhere in between...