We use a lot of QFNs, and manufacture some for our own use.  We have stopped using open vias -- no matter what the manufacturer says, the soldering is inconsistent and adding solder to compensate for wicking away can lead to the other issue of the ground pad lifting the part and causing opens.  Overprinting the groundpad (or pinpads) can also lead to connection between ground pad and pin pad solder and siphoning of the pin pad solder into the groundpad joint, causing opens.  We use the fill with epoxy and cap plate over method (which we call vippo, via in pad plated over).  This adds to the cost of the board because there are additional drilling and plating steps, but we get consistent results, better quality and the heat transfer is better.  We don't plate-shut , which I would guess is pretty specialized and expensive for thru vias.  Dave's followup sounded more like vippo.   We never use the cap one side method.  We have reliability concerns with trapping acids in this blind hole.  The issue with voids is another good one.  Definitely fewer voids with the vippo method, but it can still be a problem.  Optimization of the paste pattern and process help.  We have not tried the design to vent via diagonal solder mask presenting in a paper at IPC2011.    --Julie

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