If you're boxing eight or fewer accounts, one B.Net is the way to go.

I'll broadcast the log in information to all accounts.
Disable broadcast, and push enter.
If there's an authenticator required, enter that.
Once it's entered, broadcast and press enter for the others to log in too (without an authenticator requirement, since I just verified I had it).
If there was no authenticator, enable broadcast and press enter immediately to get the others logging in too.

The only drawback so far for me...
I'm in the process of RAF, for 10-boxing, which means two B.Net accounts minimum.
All five of my guys with 100K achievements are on the first B.Net.
In hindsight, I should have left at least one of those on the second B.Net.
Which would mean Vial of the Sands and <Of the Horde/Alliance> on every toon for both factions.