Sheldon Brown used to say that just qr skewer compression was really adequate to hold the wheel in position with no axle sticking out so 2.5 mm of axle protrusion at each end is really plenty. This is quoted from his former web site: "In practice, the axle can be quite a bit shorter than this...even 1-2 mm protrusion past the locknuts will suffice to locate the axle properly, so, when converting a hub to the next wider spacing, it is usually not necessary to replace the axle."
I did something similar with a 126mm hub to make it fit in a 130mm frame and the wheel had no problems staying aligned.