Both. You should follow up when you are wait-listed to say, "Hey, I still really want to go here," and then confirm that again with a follow-up just before May 15th, when there will be a lot of waitlist movement. And, if you have original content to contribute, follow up periodically between then, too.
Most schools have a specific policy that they state during interview day. Some will tell you readily; others, you may have to ask. For example, Albany was very insistent that they did not want updates between the interview and decision, Georgetown gave a paragraph-by-paragraph overview of how to structure the content of an update letter, and Wake Forest said updates are welcome throughout the cycle if you have something meaningful to add, as opposed to a "Hi, I like your school a lot" type of 'hollow' letter of interest.
