I'm paranoid about eating anywhere without a plan, as my ds and I both can't eat gluten!I've never had a problem getting walk-up at any Universal restaurant, including Mythos. Disney has trained people to be paranoid about these things but it really isn't a normal part of a theme park experience.
I think the only think I ever made a reservation at Uni was for the Palm on a Saturday night. You may need one for character breakfasts. If you want to eat at Mythos- make one. 2 months out is acceptable. Uni is AWESOME when it comes to NOT having to plan every moment of your vacation- you can actually eat what you feel like eating that day
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