If using a boring machine or electric drill with feed screw I drill the pin hole after the mortise is cut, otherwise the mortising bit might lose its draw when it hits the pin hole. With a chain mortiser it doesn't matter and I'll usually drill the pin first, mainly to avoid a chip tearing out when I break through the first mortise wall with the pin bit. I don't think bit drift is a problem as long as you're reasonably careful to keep the bit square to the surface with a mirror or square. After all, the bit is straight and the pin is straight; its unlikely the bit will bend. If you're careful you shouldn't be that far off your drawbore on the tenon IF you're drilling from the reference face and your mortise is closer to that face (as in edge square rule). Actually, drilling the pin hole after the mortise gives you the opportunity to REsquare up the bit if you've drifted a bit at the start.