Basic guide is "what do I NOT want to be reaching for in the heat of combat"? That is what you NEED on your stick.
After that it is space available for buttons for what you want for convenience sake.
I have WEP, Flaps, Zoom toggle, ZOOM in/out, views, Dive recovery flaps/dive brake, secondary weapon toggle (as NEEDs), and then Clipboard, map zoom, Combat trim toggle, mic switches for radio channel (s), gear and few other as "nice to haves".