Probably not practical for "inexpensive" CNC machines but if you add another axis to allow the cutting bit to be tilted, you can mill a square corner with a V-cutter. The bit is tipped such that one side is parallel to the cut direction and the tip is in the corner. Now you can cut square. And technically, it still isn't square, the tip of a V-cutter is till a bit rounded but close enough for government work.
An alternative to tilting the bit is to tilt the work piece but ultimately this all means more steps, more complexity and more cost so I doubt it how its done in production cabinetry.
p.s. - my post is referring to INSIDE corners. If you mean an outside corner, you will get a square corner as the bit transitions from an "X" travel to a "Y" travel because all the cutting happens just at the little tangential point. However due to some slop in the equipment it can and probably will round over a little bit.
Last edited by rwyoung; 05-01-2010 at 10:57 AM.