I've written a detailed guide with downloadable maps and gpx tracks, maps of campsites and hostels etc.
http://italy-cycling-guide.info/cycleways-cycle-routes/via-claudia-augusta/
Beyond the appeal of finishing near Venice, there's no particular reason I can see for doing it North to South as opposed to the other way round - although there may be a bit less climbing involved..
The main reason for doing it north to south is during the summer there are two buses that take bikes that go up the two passes.
Hence the reason why a lot of silver haired Dutch people on sit up and begs who do the route, as there are few hills other than those two