I have a Kensington Expert Trackball which has four buttons, plus the lower two and upper two when pressed simultaneously, for a total of six functions.
I’ve been using it for years and years, and have experimented with different ways to use the buttons, and I always come back to this simple config:
lower two: middle-click
upper two: unassigned, to hard to reach.
Anything more specific, like transport commands ended up being too specific, and would get in the way eventually. But ctrl-click and double-click are super useful. So, yeah, like Nickeldome is saying, horses for courses.