Rob,

I square up the Domino Mortises and leave the tenon with cut shoulders.  I find it easier to do a little chisel work than router table or sanding.

Jack

IMHO, I don't think that the sides add much strength. The lion's share is from the top, and bottom.

I base that on the fact that the Domino has wider settings (position 2 and 3) for the cutter.

And it is recommended to cut only the first Domino at the regular width, for alignment, and all the others at a wider width, for ease of assembly.

I don't think Festool would have designed it that way if the integrity of the joint would be compromised by very much.

In a traditional M&T joint, all of the strength comes from the glue surfaces on the cheeks and the mechanical joint of the shoulders ,and none is expected from the short (don't have a name for it) part of the tenon. And if you think about this makes sense, as that would be an end grain joint anyway which has ZERO strength in a glue joint.

When I used to do M&T the hard way, I'd rout my mortises with rounded ends, then chamfer the tenons so they'd go in and miss the curves, but never glued them.

However, given the tight fit of the domino's on the narrow setting, I'm sure there is some *mechanical* strength added.