There's absolutely nothing wrong with machine printed return addresses. Why write out one more thing that really doens't matter if you can avoid it? You wrote everything else and that's what matters.
Not to mention, if the card needs to be returend to you for some reason (ie, incorrect address or it gets lost, whatever), its probably easier for the post office to read a printed label than handwritten addresses, which, when put in that little corner, often appear small and illegible due to size restraints.