Most of the time easier to clean them on the bike, I think. I just took apart my cassette Saturday (as part of a class). Some of the cogs (3-4) come off individually (and thus are easy to clean), but a bunch are attached together so you have no particular advantage over "flossing" while it's on the bike.
That said, I was doing an "overhaul" class which is a once-a-year procedure for most where you take things apart, check and clean them and put everything back together.