End grain orientation is pretty simple. If the boards have curved grain across the ends, rather than fairly straight up and down, the board may have a tendency to belly, usually, opposite from the center of the tree. If you alternate end grain, the panel will be a little wavy, if you don't, the panel as a whole will bow. Sometimes you prefer one over the other.
Cabinet grade hardwood lumber is typically FAS (firsts and seconds) which means one face will mill out with no defects (first quality). The opposite face may also mill out defect free but doesn't have to by the grading standard. In cases where only one face is the better one, usually folks have that as the show side. Since many panels are attached or captured, I'd rather have it act as a whole rather than come out wavy, i don't alter growth rings due to this. There isn't a hard rule, it all depends on the individual boards, what looks best to the worker and whether the maker feels that bellying on individual boards will be an issue as time passes. In many cases, especially with narrower boards, it may never be a problem. However, the alternating grain theory gets mentioned in a lot of books and people just take it as a rule.