OK. So time and again I've seen the same order of process for cutting a joint, in books, videos and descriptions online:

1. Cut tenon
2. Cut mortise (these two often reversed)
3. Drill peg hole in mortise
4. Drill peg hole in tenon.

Step 3 inevitable mentions that when the drill bit passes through the mortise you should take care to assure the alignment because the bit can move at this point.

This was the method I learnt from reading Tedd Benson's and from Jack Sobon's books and from countless videos on YouTube. And then I did my CAP here in France where we told to also drill the peg hole before cutting the mortise because otherwise the drill bit can wander as it passes though the already cut mortise. And this seems really obvious although I feel somewhat daft that I hadn't figured this out for myself while cutting the joints for our frame.

So I'm just putting this out there for those people who might (like me) have learnt one method and never questioned it. Drill your peg holes first. Then you don't have to work so hard to maintain the alignment and your draw bores will be smoother and more consistant (consistent difference in spacing on each side of the tenon.

Just my 2 €c... it's been bugging me for a while.