PJ:

If you want the 5B setup, go with the receiver, 2 5B's and the 12D to start with (2.1) setup.  Then add a center.  Then the surrounds.

As someone who doesn't have a center speaker at all (yet), I would say add the center last. My receiver mixes the center into the left/right perfectly fine and I have not missed it. On the other hand, I think I would really have missed the surrounds as the "ambiance" they add to movies is huge, at least for some of the movies that seem to take good advantage of them.

Just my marginally informed two cents.

Cheers!