XtheEdgeX: You are correct. A multi-wire branch circuit that serves entirely independent loads is not _required_ to be protected by multipole/tied breakers. You don't even have to put the breakers next to each other in the panel.
The 'common wisdom' on DIY boards is that while not required, tied breakers are a very good idea on multi-wire branch circuits. So if you are doing an installation somewhere where you suspect that non-_professionally_ qualified individuals will ever muck with the system, you should probably consider using a tied breaker as a design choice. (I don't want to start a 'DIY' good or bad discussion, simply suggesting that if one suspects a chance that DIY will happen in a particular situation, that one make slightly different design choices.)
I would also suggest that putting the breakers next to each other is a more 'professional' way to install a multi-wire branch circuit, and that somehow paring the hot conductors that share the neutral (say by tying them together near the breakers) is a good little note to leave for the next EC who opens the panel.
-Jon
