Charles City cancelled theirs for tomorrow night. I wish they would have had it, but I can certainly see why it is getting cancelled - there is always a greater sense of responsibility if you are the host. So, the call makes sense.
I think it largely depends on the coach. Had we competed tomorrow night at Charles City, our runners would have been under strict instruction to basically run threshold pace for the first 2/3's of the race and then feel free to pick it up from there on in if they felt good. If we had kids that do not handle heat, we would not run them. If we saw kids that did not look good, we would stop them. We would never go to a meet on a 95 degree day and "race" as hard as we could. That is not going to accomplish anything at all productive, and there is a chance it could be harmful to the kids.
However, temps are really no different now than they are all summer long for baseball, softball, early practices for CC and football before school starts, etc. You have to use common sense and always try to be a little too cautious. But, just because it is hot does not mean that you can't do anything at all. There are still ways that you can have a productive night and get some experience without "going to the well" trying to "win" at a time of year where it just does not matter.