I wish I could say reservations are a must. Mythos deserves to be packed all the time. The reality is that the most you can expect is a short wait at busier times.
Here's a great tip (IMHO): When the park is open late, call and reserve the "last seating" (They'll tell you wehen that is. Linger over your dinner and then, on the way out, you'll practically have the park to yourself. Port of Entry at night with just a few people around is magical!