At Heartwood school yesterday, I asked a timber framing instructor which order he does his mortise peg holes.
His comment that if you bore the peg hole first and then the mortise you don't have to clean out the "blow out" that happens in the mortise when the peg hole bit breaks the first surface of the inside of the mortise...
I still stand on my original post comments that this is not a historical or contemporary common standard.
I would also suggest again the amount of work to drill through more wood (though perhaps not Jon's example of timber size...???) is going to be more labor intensive than placing a sacrificial block of wood inside a mortise to mitigate "blowout" (which is our standard when really worried about it or if a person is having a challenge with this happening and/or just pay more attention to the force that is being applied during the boring process.
Sharp, properly maintained and used drill bits (T-auger or modern) should not "blow out" the inside check of a mortise at all or badly enough to be a concern...nor require any significant clean up. I would suggest that if this is taking place it is an operational error and not a need to change or adjust approach modality...
Just my observations on the subject, but still an interesting topic to explore...
