when should transmission be serviced/flushed

I wouldn't necessarily go through the flushing process but dropping the the pan, replacing the filter and fluid would suffice every 30-45k...If you decide to flush, make sure it's only flushed with trans fluid, without any additives and that the trans cooler is flushed at that time, also. I've heard of too many nightmares using blended solvents.

For optimal performance and longevity of the transmission, every 30,000 miles is about right.