At any live cash game (at least in Minnesota where I play most often) you can much your hand at any time - Regardless of whether or not you are all-in. While it would be rare to see it happen in such a rare situation - It happens quite frequently here. Most often it will happen in lower stakes games when a husband/wife are playing together. If it is just them heads up and one is out of chips - Often times the winner will muck their hand just so that the loser gets some chips back.