I think technically breakfast at both the Three Broomsticks and the Leaky Cauldron are Reservation. But, reservations are only taken for people who book a special HP package. Otherwise, everywhere in WWOHP is counter service with no reservation required. Even at breakfast, walk up are accepted most days.
Unlike Disney, even the table service restaurants in general don't require massive amounts of advanced planning. If you're traveling during extremely busy times you may want to book a couple of days in advance. For example, Mythos at IOA seems to be booked up for tomorrow but you can get a table for the 28th. But Finnegan's at USF has tables for tomorrow. Emeril's has lunch but no Dinner.