Yeah, with them turning "stand-by" guests away on a daily basis, it's best to have priority seating arrangements made well in advance.
And I wouldn't mention the fact that you just want to go for dessert (if you know what I mean). They don't need to know what you will be eating when you make PS's. Or worse, if you are trying to get into the stand-by line (if there is one).
Besides, you just might see something that you like being served next to you and order it.