I write checks for Mama and my uncle all the time. If, for instance, you put $24.00 in the box for the number, and then write out 'Twenty and 00/00' on the legal line, the bank is bound, by law, or used to be, to cash the check for the amount on the legal line.
That doesn't answer framers question, though. My best guess would be that, legally, as long as the client SIGNED the check, and it was someone you knew and trusted, it'd be ok.
I wouldn't do it for a stranger, as sue-happy as people are today. They could always clain you quoted one price, and made the check out for a greater one.