Zero for the group that I play within, and that goes for pickup games we play as well. Seems a bit tight maybe, but I/we do allow opponants and ourselves to be under by as many points as you want. For us it all comes down to being realistic and losing that couple of points somewhere to make it fit, you can't always have everything you want. Mostly it's between 1 and 3 points under, for example.
Couldn't be bothered to read the whole thread and give argument on cases brough previosly.