Being from a northern climate, I find electrolytes to be absolutely crucial when travelling to southern shows. It's extremely dry where I live, and I have found that it's not so much heat alone that bothers my horses, it's humidity that's killer. My horses have been just fine in 110 degrees in Reno without electrolytes, however at Worlds a few years ago it was unusually humid. Even though temperature-wise it wasn't super hot, the horses were just exhausted until the last couple days of the show because we made the mistake of not starting them on electrolytes until we got to Texas. By the time they got back to a normal energy level, they were done most of their classes and it was too late.
So now we start the horses on electrolytes a couple days before leaving home and keep them on them throughout the show, and we haven't had a problem since. I use the powdered electrolytes to top dress their grain and they work great. I also make sure to bring as much water from home as possible - obviously for a long show like Worlds it won't last even close to the whole time, but by the time it runs out they are drinking well and don't mind the switch - and the continued electrolytes encourage them to drink as well.