overdrive is meant to be used when driving normally.
Its a higher gear so that, at cruise, your engine RPMs drop so your not sucking fuel down like a drunken gorilla.
Turning it off is just for those times you need higher end power (towing, or passing)
Otherwise, you push the pedal to the floor, and it'll shift out itself...or, you can just push the button to disengage it.
NEVER TOW WITH THE OVERDRIVE ON!!! Besides that...leave it in