The stock blower has a sort of bypass valve that will bypass the blower at light load. This avoids stress on the blower and lowers inlet temps when not in boost.

I don't think anyone is putting a true blow-off or bypass valve since it would have to go after the blower and there isn't much room to do so.

On the centrifugals (Vortech, Paxton, Pro-charger, etc) blow-off's and bypass's are much more common than they used to be. Its a pretty good idea to run one.