[mark houghton]

Everybody has covered it pretty well. Generally speaking most would say don't do it, due to the rich spike it will cause, but a lot of people do it anyway I think...if not for anything but to get a woody when she 'whistles' between boosted shifts.
Not that it's damaging or anything, just goes pig rich when you jump off the throttle to shift, plus you lose the volume of air that in a normal recirculation valve setup would still be available when it comes time to get back on the gas and boost her up...supposedly resulting in less turbo lag.