Since my build thread, Matt has confirmed that the 'preferred' place to pin the boom is the front clamp. However, he has yet to see, or hear, of a failure of the boom in flight with a rear clamp pinning.
I looked over my other helis (I have 14) and my Gaui X7 & NX4 are pinned at the rear clamp, as are all 4 of my Aligns (550, 2x700e, 700n) my brand new Thunder Tiger E700, and my N5c.
That lot adds up to a lot of flights, and I've never had an issue, and I've never seen any top level Align pilots break one in flight (at the front clamp) and I know 3 factory pilots personally.