This is a very good question. The truth is , it doesn't really matter. The act of brushing your teeth polishes the enamel and removes pieces of food from within and between your teeth. Of course, if you don't brush your teeth for a long period of time, then these food particles are broken down by your saliva and the resulting components can contribute to plaque, tooth decay and discolouration. Brushing also can help with halitosis so I guess it depends on the meal you've just eaten as to whether you should wait or not.
Happy brushing.