Another factor to add is the effect of drawboring the joint. The draw bore not only pulls the joint together during assembly and as drying occurs, it also puts you on the side of correcting error. If you mark out your holes exactly and one is off one way or the other it can in effect push the joint apart, so we ensure a correct pull to the joint by forcing the joint to come together with a draw bore. Even if the mortice hole is bored a little off the draw more often than not, I don't think I have ever had one push the joint apart when done correctly, will allow the joint to come full together. Preassembly and pricking the tenon through the mortice peg hole will give very accurate peg location regardless of the angle of the mortice peg hole. Why do we feel the need to be so exact in some small aspects of our work? There are many ways to over come such hang ups. I never bore the peg hole before mortice work. Maybe if I was using a chain morticer, but even then I wouldn't.