If you wish to have them in it at reasonable time after you arrive, you almost have to make the mad dash at park opening, or get a dining reservation prior to open. I have seen the line wrapping every which way even twenty minutes after park opens. Example, sister and brother in law arrived before park opening, made a mad dash run to the sign up. My nephew was scheduled for 4pm something ...
The biggest kick is the park is essentially a half day park. If your willing to go get signed up and come back, that's one thing, but your loosing over an hour or two doing that (travel to park, sign up, travel back). Otherwise getting a late afternoon time is a bummer, you will run out of things to do by noon.