Quoted from DJKYrtgfsRFw:
1) You don't need to 'level' the upper playfield. If you level the machine correctly then the upper playfield will level correctly and balls will NEVER get stuck in the corners. I'd love to flying clothes-line the person who put in that screw
Not quite sure what planet you're from, but WMS specifically added a leveling screw/spring to the bottom right mount just for leveling. It is often necessary, since the posts and side rail's heights may not be exactly matched.