My tractor needs to be in 4 WD about 95% of the time. The Catskill Mountain land here isn't all that steep, but it has enough varied terrain so that there always seems to be one tire a half inch off the ground. 4 WD is needed for most routine tasks here. In winter, 4 wd is on 100% of the time, and the differential lock and the loader-as-paddle get their workouts too.
The only time I try to remember to take it out of 4 WD is when I'm using it mostly on our (gravel) road or those rare instances when it hits pavement.