Loose grip....

[saxman]

Shim it, but find a washer that fits the screw and screw head exactly. Blacken the washer by blueing if it's plain, uncoated steel, brass or copper. Black plastic ones can sometimes be found, or plain nylon. You may want to blacken the edge of nylon or clear washers with a 'Sharpie' laundry pen so they're not easily seen.
Another way is to carefully build up the screw contact area on the grip with superglue. Clean the screw contact area on the grip with lacquer thinner, then apply some 'Loctite' liquid super glue (not gel). After a couple of hours you can check the fit, but it will probably take several applications. A benefit of this method is that the watery superglue soaks into the wood and hardens as it cures, making that delicate and critical area much stronger. Find 'Loctite' super glue at Wal-Mart in the new, well-designed applicator - the first one I've seen that really works.