In the following images I've animated a button dropdown where the arrow animates to the downward position when displaying the dropdown menu.
Wondering if there is any usability studies on this or if it's mere personal preference.
You can see the animation here if it makes any difference https://dribbble.com/shots/2369431-Daily-UI-027-Dropdown
IMHO I think that the arrow should point towards the content. So when the dropdown is active, it makes most sense for it to point down, when the content goes away, it points up, where the content came from.