Even when the box/sensors are perfect, there still is a slight hump when cold.
When up to speed the od will engage when it see's fit, leave the od ON, ie NOT illuminated.
The only time really it's useful is in slower moving traffic, hills, as they are a bit reluctant to change down to 3rd lock or slip, the 4th is a direct lock, as you become more used to it you will soon see when you benefit from turning the od off so that the light illuminates, always before overtaking a car etc is a good time, saves the pause from the box to change down, although it will be instant in kickdown(full throttle).
You can't do any harm by leaving on all the time, you just won't get the most from the box, most don't even realise it has a od off button!!
Don't be afraid to use the revs, I would hit the limiter in every gear in mine, had 137mph indicated once, private road of course.
There are far worse auto boxes to have, and most of the early problems have been sorted years ago, you can even get shift kits for these too, or even make PADDLE SHIFT, easily enough.
tabetha