Dunno, I've seen many 'Q.C. passed' stickers on my PSUs so I assumed they were actually tested.
Just out of curiosity I checked my pile of used PSU's: The one I trust least had the most Q.C. stamps on it! You may be right, though, that they actually are tested. The question is then,
how intensive the testing is in high volume production. This document:
http://www.silabs.com/Support%20Documents/TechnicalDocs/AN700.pdf, although not being about PSU's, describes quite thoroughly the level of testing during various phases of manufacturing. Chapter 3.3 on page 10 nails it:
The hardware design and manufacturing process have already been proven, so the product now just
requires a quick “go/no go” functional test to verify operation.
I wouldn't be surprised if the Q.C. stickers only indicate that random items get tested. Time is money, and plugging each connector of a PSU to a tester would take several minutes. Then again, this page:
http://www.cougar-world.com/psu_production.html claims that every single PSU is hand tested. Even that won't count out the possibility of human error: In a hurry it's easy to confuse a defective item to a long row of faultless ones.