To me, putting glue on the domino is so perfectly obvious that I couldn't understand how so many could be putting it in the mortise. I was going to contact Festool and get their opinion, when I remembered I had their extended manual on PDF.

When applying glue for the tenons, you can either apply
glue into the mortise slots, or spread a thin layer across the
Domino tenons. For applications where the tenon is the
primary structure holding the joint together, you should
apply the glue to the tenon. The Domino tenons have small
glue pockets and ridges that will hold glue as the tenon
slides into the mortise slot.

So, each to their own, but putting it on the domino is best.