I like one B.Net account, for each account.
That lets you log in all accounts at once, with a single authenticator tied to each.

It also forces you to log in, one after the other (pre B.Net style) for account management changes.

There were rumors things would become B.Net bound, instead of Account bound.
If this ever becomes true, then splitting the accounts across multiple B.Net's is another disadvantage.

If you put all the accounts onto one B.Net.
You can log into your B.Net once, and then (via drop down box) switch from account to account for changing options.
But it takes 30 seconds per account to log into wow (with an authenticator).