Overdrive should be used only for highway driving, actually. If most of your driving is stop-and-start, or under 40 mph, you're really harming your transmission. If the car constantly shifts from drive to overdrive and back down again, that will lead to premature transmission wear and failure. You can feel at exactly what speed your car shifts into overdrive (it varies for each vehicle) and if you will be traveling near or below that speed, lock it out of overdrive so that it doesn't shift unnecessarily.
Using the overdrive regularly, especially when you are just cruising or driving over short distances is not recommended. Overuse of the overdrive causes transmission wear; use it at highway speeds of over 55 mph or when you don't expect to keep downshifting.