Prior to me selling my Trans Am, I did full flush but I maintained my tranny very frequently. It really depends of if you have been maintaining your cars fluid or not.
If you have been flushing or draining every 20K miles as flush is safe. If you have neglected the transmission, say 35-50k miles between fluid changes, then if you do a flush and the converter is full of sludge, the tranny will start slipping after some time.
I'm not trying to scare you, I know from first hand experience. My prior car (84 TA) I did not maintain the tranny very often. This car that I owned for 6 years had a flush with no problem what so ever. I also added a B&M trans cooler for safety and keeping the fluid safer, for a $100 it is worth it. Most dealers would rather flush because of cost and it is easier, saves on dropping the pan and prevents pan leaks.
Hope this helps and this is my opinion from experience.