If you use or plan to use an authenticator...
Then one B.Net per one WoW will let you log into them with a single code.
So you can get in, in the same time it takes you to log in now.

If you don't use an authenticator and do not plan to get one.
I'd definitely go with a single B.Net account because the only drawback is the log in time with an authenticator.

If you do have an authenticator, and...
If you don't mind a 30 second wait between each account logging in.
Then go with one B.Net for all your accounts.