I did some investigating. The latest rule, Check 21, stll requires origial checks to be printed in mag ink. But somewhat ambiguously, it also allows a "substitute image" to be processed as if it was the actual check. Phone aps read checks using OCR, not MICR. So I think it would be possible to print a check on a regular printer and use your phone to deposit it. Whether or not it's ok from a legal point of view, I don't know.