If you are really flexible about when you want to eat and don't mind waiting, maybe not. However if you have a rough plan of where you are going to be it always helps to secure the time you want too. Even that morning or a day or so in advance helps, purely because it means you aren't left waiting 40 mins or so (any time around 6/7 pm or breakfast time 8/9am is going to be busy and you will wait) Really does sort of depends on how long you are prepared to wait. Off season in October we have had to wait 40 mins with a walk up. We were able to get good PS's the morning of, or the evening before. 'Ohana's is probably more in need of a PS (or advanced reservation as they are now calling themselves) at breakfast then at dinner. Rose & Crown is always a popular spot around Illuminations time.