I think warm-up is very important both before and after you ride. Like pmweeks said, it's like if you were expected to suddenly do a sprint run without stretching, walking or light jogging first. I always walk my horse about for about 15 minutes before starting. I always do a lot of trot work before canter work. Canter work always comes at the end. Its good to walk your horse out after the ride as well. I'm sure you know what its like to run around a lot and then completely stop, its the same with horses. They need to be eased back into a walk, not just stopped completely. Sometimes you won't need to do much. You can tell if your horse needs to cool off by feeling his chest and neck for heat, and also checking the colour of his nostrils. If inside they are red, he's probably a bit hot.