The show will pretty much always start with a "party starting" dance. An uptempo latin or a quickstep mostly. The last routine will either be very good or a big spectacle that requires cleaning up afterwards (and they do try and share it around the "contenders" as much as possible). Generally the dances will get better as the evening goes on as the show builds to a climax, but it's not exact. If the show has a big name who's getting them a lot of publicity (such as Peter Andre last year or Ed Balls this year) they will tend to go on in the second half, especially early on in the series to avoid the drift to X Factor. They'll also try to organise to avoid having too many of the same "type" (ie "the pretty young girls", "the comedy contestants") on one after the other, and the same with different types of dance (ie they won't have a waltz followed by a waltz). Other than that it's probably just what works in terms of setting things up and taking down sets.