We ate at Mythos twice during our trip this past week and I had reservations both times for 11:30 AM (this is when they first started seating, though they opened the doors a bit earlier - not sure exactly when). Both times I made the ressies in the morning, probably around 10 at the dining kiosk in the park...once at IOA and once at US. The first time (a Tuesday), when we got there around 11:25, there were already people there and about 10 parties already signed in. We were seated right away (first in the restaurant) and one guy actually came up to the podium and complained! The hostess explained we'd made a reservation. So yes, I would definitely advise doing this for whatever time you'd like to dine. When we left there were even more people waiting. Mythos is deservedly very popular!