A good rule of thumb is any schools that require them at all, tell you what LOR's they want, and don't state specifically that letters are required with initial application, by interviews, or by acceptance, etc.
So if it doesn't say specifically when then assume they want them before secondaries.
When schools evaluate your app most do an initial screening of gpa/mcat and other requirements, often right before or after sending you a secondary. Then once they get your secondary they send your completed application on to be reviewed at the next level. So at that point you are potentially wasting their time if something is missing. The big exception is some DO schools don't require a DO letter until acceptance.