Motor temp is one of those subjects where there are a lot of variables. The biggest variable is where you take the temp. With Nitro, you always shoot around the glow plug. With electric motors it's not so simple.
With sedans, I like to shoot the can near the vents, but when I race offroad, that isn't possible, so I take the temp off the brush hood area. The can is more a measure of how hot the MOTOR as a whole is. Brush hoods get hotter when you are running, but they also cool off faster after a run.
I've gotten sedan motors up to 235 and that was WAY too hot (running stock 6-cell sedan on a parking track in the middle of summer) -- that will hurt a motor (magnet fade, etc.). I normally try to keep temps around 160-170. That seems to be a little conservative, but the motors hold up well. In general, I'd say keep it well below 200 for sure, and if you are down around 130 or so, you are leaving way too much on the table. Keep in mind that the magnets in Sagami based motors (Yokomo, Reedy, Orion, etc.) seem to be much less affected by heat than EPIC based motors. I've "cooked" Reedy and Orion motors by gearing errors, but once they cooled down, they were back to full performance. EPIC motors have magnets that can fade under the best of circumstances, and if you "cook" them, they must be rezapped and may never come all the way back...
No magic answer, but that may help some...