I wouldn't recommend turning off O/D unless you're in a situation where the ATX will "hunt" for a gear. For example, there's a long uphill onramp on my way home that has a stoplight at the beginning. If I leave O/D on while I climb up to 60mph, the car will often times up/downshift between 3rd and 4th because of the incline. I turn off O/D until I get up to speed, then switch it back on.
Maybe Phil or someone else can comment on this, but I think that leaving O/D off during normal driving will probably result in higher temps in the ATX, which isn't good considering its propensity to overheat to begin with. Plus the TC really needs to lock/unlock when appropriate; forcing it to be locked constantly couldn't be good for it.
