Not necessarily - to test this, I've just looked up pictures of various double acts: Morecambe and Wise, Vic and Bob, Hale and Pace, etc., and they vary which side they stand. AFAIK Ant and Dec have always stood the same way, and now Holly has taken Ant's place.
Anyway...
The most important thing is that we still don't know who's gonna wash 'Arry's pants...
Think Holly is doing a good job, last night's show demonstrated she is starting to settle a bit. Dec (and it IS Dec - easy way to remember is that they call themselves Ant & Dec, and Ant is ALWAYS on the left...