John -
Your owner's manual ought to cover this. Find it online if you don't have a copy, but it is probably in the glove compartment.
For my Ford, the owner's manual says to turn it off to prevent excessive shifting (also known as hunting). This will prevent the transmission from shifting repeatedly amongst 6th, 5th, and 4th gear as I go up and down minor grades, by locking out 5th and 6th gear. It is the act of shifting that is hardest on a transmission, especially if it is done under load.
So again in my Ford, I leave it alone if I am on the flats and moving along nicely (65 mph or so). The tranny locks up in 6th gear, and stays there. But if I have to slow down, or if the terrain gets hilly (not mountains, or even steep hills - just a bit hilly), I will notice that the tranny starts hunting for the right gear, and I hit the button to prevent it.
Bill
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