Outside of the smaller venues (comedy show and maybe the ice skating show) reservations are only needed so you can get seated early. They allow you to begin sitting 30 mins before the start of the entertainment. 10 mins before the show starts, then non reservations can be seated. Think of this as getting a preferred seating FastPass at WDW, but rather than having a predefined area to seat at, you get to seat earlier.
Even if reservations are fully booked for a venue, there is a good chance a seat will be available.
If you have flex-time dining, it is similar. You an reserve a time to eat in the main dinning hall, or wait in a line to be seated. If you have a reservation you have priority seating over those in standby for eating. However, I have never seen someone in the stand-by eating wait more than 20 mins to eat.
This is not Disney World level of planning. If you forget to or cannot make your reservations until you board, that is okay too. They will have kiosks set up as soon as you board where you can schedule out your reservations. They will have the kiosks available throughout the cruise, but they have a lot set up before set sail.
I have been on several Royal Caribbean cruises and I have been on the Allure of the Seas once before (it is the sister ship to Oasis and pretty much the same with minor differences). Let me know if you have any questions about the ships, entertainment on board, etc. There is a good chance that I have already done it or know about it.
we just finished watching the 10th Anniversary show, we look forward to seeing one ourselves 
