I don't know why people think they need a reservation at Mythos, because most of the time, there is no wait. I've never heard of people saying there is a huge line to get in. I went last July 7th at noon, walked up, and they had a table ready against the window.
Since they don't stay open late (unless it's the summer), the best time to go is for lunch. Take the left overs back to your room, and snack on them later.
If you ask me, the places where you would need a ressie would be Emeril's, Tchop Chop, and to arrive the Hard Rock Cafe at 11am for lunch, or 5pm for dinner. Margaritaville is probably packed too at peak times, so show up an hour before.
I dunno, maybe I got lucky each time at Mythos, but if you ask me, if someone walks up and the wait is too long, they're probably going to continue on to a Citywalk restaurant.