Well said Leopuppy, I would not put any of my rotties in a trial in those conditions, black dogs just can't handle it. I wouldn't take them to a conformation show either, but certainly a normal all breeds show would be way less demanding than an obedience trial. Usually only about 5 mins in the ring, a little triangle to trot around, look pretty and thats it!
I would hope most people would use their common sense, and depending on the breed, and the individual dog make a decision on whether it was good for the dog or not.
I think an agility and obedience trial would be much more taxing than an all breeds show, and therefore the cutoff in temperatures could be a little higher.