From my limited experience, draw boring becomes harder to accomplish when your braces (and tenons in general) are toward the middle of your timbers. Any misalignment of the boring bit (deviation from 90 degrees) becomes amplified the further you bore into the timber. (and if your timbers are not exactly square, and historically they would not have been, then this is even more of an issue). Not sure if this is _why_ they held braces to the sides of the timbers, but this is one reason _why_not_ to hold them toward the middle. Of course this would not be a concern if you do not use draw boring.

-Thomas