Fred,
I have always limited my transmission temp to 230 even though I know the transmission brain is programmed to shut down the retarder if the temps reach its limit. My reasons were solely to be conservative to protect the transmission fluid whose life is compromised by higher temps.
Having said that I have learned techniques to allow driving in mountainous areas while minimiziing temperature gain, and they were used and proven in the Flagstaff area on a trip to Pahrump NV.
First, using two notches of retarder on my coach provides me with a downhill run on a 5% grade with a less than 230 degree temp and no increase in my set speed. When the grade is steeper I slow to below my 4th gear max speed of 55 and engage the retarder and fourth gear. I used that on 7% grades out of Flagstaff and the hills near Henderson NV. I never had to use the brakes and if the speed started to approach 55 I went to the third position on the retarder control. Again the temps were held to just below 230. On the steeper grades, in fourth I moved the retarder selector between the second and third positions to regulate my speed to just below the fourth gear limit of 55 (2100 RPM).
We are driving heavy vehicles and downhill runs have to be done under control at lower speeds. I was initiallly concerned about truckers being pissed because I had dropped below the speed limit until I realized they were doing the same thing. That is a western thing however, because on long steep grades in the east I notice truckers at or above the speed limit fairly common. I have stopped caring what they think of my speeds.
Using the retarder is a balancing act. If you leave it on all the time just heavy braking is going to cause your temps to spike. You have to be vigilant, avoiding heavy braking by anticipating and slowing. When you have an emergency, that is when you disregard the fluid temp spike and concentrate on braking, but by being ahead of the bus at all times you can avoid that need.
Or you can just drive the bus at car speeds, ignore the fact you have a 46,000 pound weapon, and push your fluid temps in which case your transmission fluid will wear out quickly. If you look carefully at the Allison maintenance schedule for fluid changes you will note that retarder equipped transmissions require fluid changes twice as often as those not so equipped. The reason is the heat.
BTW, the heat does not get to the transmission. The temps you see are measured at the retarder output just prior to the coolant to transmittion intercooler located on the drivers side low next to the engine.