My take on it....
The problem with high res temps is that it encourages microbes to grow in the nute solution. This probably will not effect healthy plants with good roots. However if pathogenic microbes are given the chance to take over, due to sick, very old, very young, or damaged plants, you could end up with a bad problem that doesn't go away, and greatly effects yield.
There are two ways to approach this situation. You could try running a completely sterile res. This is fairly cheap and easy. There are many products like zerotol, sm-90, super oxy, dutch master zone, ect that will do a pretty good job of keeping the res clean and sterile. Some people simply add h202 every few days. Anyone interested in this route should do some real research on it. I personally recommend Zone.
Some people are unlucky and due to some random factor they have a terrible time trying to keep the res sterile, and when combined with high res temps, the result is slimy, mushy, dying roots. At this point the strategy becomes trying to add beneficial microbes to the water that will displace the harmful ones. Harmful microbes tend to be anaerobic, so adding bennies and keeping lots of oxygen in the water will encourage an aerobic microbe solution, which is nomally enough to keep the bad stuff away. However, the bad stuff will always be there, especially at high res temps, just waiting to gain a foothold when an air pump goes bad, or something similar.
Some people buy their bennies in bottles or powder form. Great White powder and aquashield liquid compost seem to be favorites. A really cheap and easy way to add bennies is to make a 'tea' from earth worm castings. You'll get a freshly active, and more diverse army of bennies from that than from all the bottled stuff combined, and for a tiny fraction of the cost. Ancient forest from GH seems to be the preferred product for this. If you want to go all out you can add the aquashield, great white, and roots excelurator to the EWC tea to cover all the bases. Again, anyone interested in this route should do a lot of research on it before they implement it.
I personally would try the sterile route first because its cheap and easy. But if your like me, and most people, once a problem creeps up sterilization and then inoculation with bennies is what will fix it.