This has been debated here on TBN for years. Road vehicles like pick-up trucks are not the same as a tractor unless your tractor spends most of its time on the road.
Leave it in 4wd all the time or only when you need it all comes down to a matter of preference. There is no wrong answer _except_ when it comes to pavement.
However, I do not buy into the philosophy that keeping it in two wheel drive most of the time is conserving anything at all or promoting the longevity or reliability of your tractor. A good 4wd tractor is designed to be used in 4wd. 4wd is not an afterthought or a patch job on a good tractor. Yes, of course there is wear on the system but other components wear when 4wd is not engaged unless conditions are extremely mild.
The only downside to using 4wd all the time, that I can think of, is front tire wear. The fronts on my tractor are slightly overdriven compared to the rears. Leaving it in 4wd probably adds a miniscule amount of extra wear. But in ten years of being in 4wd all the time, my fronts have easily got another 5-10 years of wear on them. Plus, when in 2wd and turning tightly the fronts will often 'plow' so that is some degree of additional wear (and poor handling) you get from running just in 2wd.
Moral of the story: do what seems best....except using 4wd on pavement...........especially if your fronts are slightly overdriven like mine.
