you mention that this is a tie beam - so a post comes into the mortise that you are trying to peg?
does the peg in the photo point up the post?
or the beam sits atop the post?
drilling the tenon after it is assembled is certainly a safe(r) way to do this, eliminating the 'pull' from the drawbore. (or not pegging it at all, depending on the answers to the above questions)
in the end i think it is a judgment call based on how you lay out that mortise, where the heart lies, and how much check you think will run into the cheek of the mortise.
