Do it now...... Clean up the combine now, to keep mice and varmits out of the wiring harness...Change oil and all filter including air, cab air, fuel, water, hyd. I prefer to do it in fall because you never have time in the spring when you main goal is to get in the field.. Also now is the best time to check out those little things you "lived with" during harvest... If you don't do it now you will probably forget it by next spring...