Even for a piper I'd say the normal conventions should apply ie. no Prince Charlie before 6 PM. At my brother's wedding we all wore kilts with Prince Charlie jackets, the piper wore a tweed (Day wear) Argyll, he looked smashing and was more appropriate than we were. When I was planning my own wedding the question of what to wear came up and I knew immediately that I would not be wearing a Prince Charlie as it was a day time wedding, of course that was after a few years here. Having said that even my Mom said don't wear the P.C.

As for the piper being the best dressed kiltie around, I think someone who is dressed in a P.C. during the day is as inappropriate as someone showing up to a black tie event in a T-Shirt and a casual kilt.