New transmission fluids are designed to operate normally in the 170-190 range. The viscosity stabilizes and the trans shift "normal" thats why they usually run thru the radiator cooler before returning to the trans, keeps the temp in the operating range and the water in the radiator is very effective for pulling out heat.
As the temp rises, fluids will start to oxidize causing the fluid to start breaking down. This usually doesnt start to occur until the 240+ range. Up to 215-220 is putting some stress on the fluid so changing it more often is required but it shouldnt damage anything unless operating like that for long periods of time. If you dont have a lockup converter the temp will be higher than most of the 700R4 etc with lockup cruzing down the highway. Depending how hard your running is off-road is something to watch. You would be suprised at what your enging oil can climb to when your wheeling hard, etc (300+ /forums/images/graemlins/eek.gif). The post of the temp vs change interval is very good. Give it a good flush every couple years and you'll be ok.