One of the things I hate is dealing with clients who are slow in paying. I'll actually sometimes decline to work again with certain clients who are especially bad about this, and I'll give preference to clients who pay with PayPal — despite the fees that are deducted for each transaction, it's worth it in order to get payment quickly and remove the possibility of the "check is in the mail" excuse.
A couple of months ago, I had two clients around the same time saying they had mailed a check, yet neither was arriving. This was the first time I'd actually had this happen with two clients at the same time, so I was actually concerned that something was happening to our mail! As it turned out, one client had forgotten to mail the check (it sounds suspicious, but I actually believe him because he went to the trouble to meet me in person and hand-deliver it), and the other had simply lied — clients may claim that "the check is in the mail," but the postmarked date will tell the real story later on!
Do clients really think we're too dumb to note what the postmark says?
I personally don't advocate working with a client who gives you cause not to trust them, even if they do ultimately pay you. Have you heard the "check is in the mail" line before, only to get it a week or more later than you would expect? If a client lies about when they've sent payment, but they still pay you, do you still consider that grounds for terminating your relationship with them?