Actually, while most animals CAN eat raw meat, as most are herbivores it's not always ideal as a food source, especially if not fresh. Horses in particular are herbivores (but have been documented as eating meat previously). Humans have evolved from herbivorous animals and by cooking meat in particular, we break down the proteins so that our stomachs can better handle the food. Even cooking things like potatoes maximises calorie intake for humans. This is the important point; cooking any food makes it easier to digest and extract the energy and nutrients from it, regardless of whether it is meat or vegetable.
So, if your centaur has a hybrid human / horse digestive tract (this is by no means certain) and that species can digest meat, it's almost certain that it would be beneficial to cook it. That said, some vegetables may be more readily digested without cooking. The interesting question would be whether or not your centaur could digest grass. Humans can't, but it's the principal food source for horses.
Ironically enough, Centaurs with a largely human digestive tract may well not have any concerns with appendicitis. The reason for this is that it is thought that the appendix is an atrophied organ that helped primates process vegetable matter more effectively. The appendix in your centaur would probably be restored to its original purpose meaning that the organ is no longer redundant and therefore less likely to become aggravated or inflamed.