Assuming you're drilling and tapping in one setup, would be good to mill the edges of the place to everything is square.
Would also be nice to fly mill top and bottom first to keep the planes parallel OTOH depending on material you might end up with it bowing. Tooling/jig plates are sometimes made of cast aluminum, there are other suitable materials. If you're putting down a piece of sheet steel you may find it bows out of flat.
Would also be nice to fly mill top and bottom first to keep the planes parallel OTOH depending on material you might end up with it bowing. Tooling/jig plates are sometimes made of cast aluminum, there are other suitable materials. If you're putting down a piece of sheet steel you may find it bows out of flat.