[Klaus]

Thanks for the clarifications. I think I'll keep my 16-50/2.8 because actual pictures look very good, even wide open. I will however keep this in mind when buying lenses in the future and will use this method, as it's really simple to execute. The limits I'll consider as "acceptable" still need to be defined :-)

On a related question - are de-centering issues more likely to affect the wide or the long end of a zoom? In other words, if I have limited time to test a lens, which end should I prioritize?

There are no rules here - I've seen both ends with issues.

At the wide-end you tend to have more issues with asymmetrical sides whereas the center remains very good.  
At the long end, you "magnify" any issue while essentially cropping the corners. Thus the corners are often symmetrical but there's an overall softness.

But, as mentioned, there's a wild mix of everything out there.