I comment on the visual condition and operation of the windows, but air leakage is more of an energy efficiency issue which varies dramatically from window to window.
Common sense tells us newer are tighter than old and higher quality are tighter than cheap. But testing each window seems a bit far out of the SOP and clients can evaluate that just as well as any inspector. In almost ten years I’ve never once had a client show any concern over windows. I should also mention, though, that in my area the contracts the agents use specifically exclude windows.