As a general rule, you are correct -- however, I think thank you notes during the admissions process are an exception to this rule. I work closely with faculty members who serve on PhD and MD/PhD admissions committees who routinely receive handwritten thank-you's as this is considered the norm in this circumstance. That being said, I can't imagine sending an email rather than a handwritten note would count against you, I am guessing the admissions committees just want to see that you have had time to reflect upon your interview/introduction experience to the school.
