One might also speculate that while Klingons' eyes do become red ( http://klingonska.org/canon/2003-12-holqed-12-4.txt ) and even produce excess water when under great emotional duress (such as Kahless crying out a lake), they don't have an organ specifically for collecting that water into drops. As such, they would understand {mIn bIQ}, but find the expression {mIn 'onroS} a bit strange, as their tears come in more of a continuous stream.
Spock may have been under the misconception that this meant they could not cry (as Federation knowledge of Klingon anatomy and culture were incredibly limited at the time), or perhaps he was just being overly pedantic in response to Scotty's comment that Azetbur hadn't shed "a single tear". It may also have been a comment about how the Klingon response to the death of a loved one is different from what a human might expect.
//loghaD
Isn’t it just as likely that Maltz is lying to Okrand? If tear ducts are a sensitive topic of conversation for Klingons, denying their existence could be a manifestation of his anatomical shyness.