My experience goes right along with yours'.  For me, going over the combine seems to be the most "fruitful", as you call it.  Being able to look the combine over, in a no-stress, no hurry up a go mentality is the best thing we can do.  We also do this to the tractors & the planter, & the drills but it seems to help most on the combine....since the combine has the most moving parts and wins the prize for "most likely to break".

It ought to be mentioned that my combine is "old" judging by most of y'alls standard.  My combine is a JD 9600.  Yes, we have the occassional belt break, or an alternator go out, or some little gremlin like that.....but it has been a long time since we have had a major break down during harvest.  I consider our off-season work to be very beneficial.

Since farming is not the primary "bill payer" around here, I ought to also mention that changing the oil in our (18) windmills is also very beneficial.  I know a lot of ranches that don't do this (instead, they ~sometimes~ get it done when they have the mill re-leathered), but we change the oil in our mills every year.  I also mix-in a little STP into the oil.  I think it makes the oil cling better, especially to oiling ring at the top of the [aer]motor.