In my experience on heavy duty trucks, if you think it needs a flush, it needs a rebuild. Saying that, most budding problems are solved by regular servicing. There is a reason road trucks regularly rack up a million miles, they are serviced so very often. On the trucks we had that were automatic we did a complete service every 60K miles or as close as possible.
What we did to those trucks roughly translates to fluid and filter change, band adjustment, valve body partial disassembly and cleaning, solenoid removal/inspection/cleaning. Most trucks would run the same set of clutches and bands for about 200K miles before needing replacement.
I've been maintaining my parents vehicles for years, I do a fluid and filter change, band adjustment and valve body inspection/cleaning every 60K miles. The vehicles have 210K, 198K and 170K aprox. at the last time I changed the oil. The engine and drivetrain just purr on those cars and SUVs.
Don't flush it, at best it does nothing, at worst it forced contaminates from the bottom of the pan into the workings of the transmission.