If you feel like buying a new set of wrenches, you could try the Craftsmen Overdrive combination wrenches, the closed end is supposed to work on fasteners that are up to 70% rounded. Supposedly they use the same tech as the Mac R.B.R.T wrenches (since they are owned by the same parent company). Only run into a couple rounded fasteners since I bought them, but they worked great both times so far. Lowe's carries them.
