Can't speak to the Mill Creek for sure, but we did not fiberglass the puzzle joints when building our Passagemaker kit.  Had to go back and look at some photos to be sure.  We followed the manual very closely so, without looking, I'd say that the manual did not specify this, and that's probably CLC's intent.  We've not had any issues with the joints after three seasons of use.

I think the trick is to make sure you get plenty of the thickened epoxy into the joints and then get 'em clamped down tightly and evenly between broad, flat surfaces.  After the epoxy cured, we carefully cleaning up the surfaces, and the planks bent into shape when we wired her up without any unfair spots.  Heck, under the paint, there's no indication that it wasn't just all one-piece 12' planks.

Hope that gives you some comfort.  Maybe some other builders will chime in here.

.....Michael