Personally, I think putting the GPS close to the top of the windshield is dangerous. If the cup let go, it will fall from higher, bounce around longer and create more general disturbance in the car and, that could lead to an accident.
In case of accident unrelated to the GPS, the high-mount unit become one more projectile who might hit somebody. The lower it sits, the less chance it will hit somebody's head and the more chances it will get stopped or slowed-down by the car interior's first.
Mine is stuck to the windshield, in the center of the dashboard, as low as it can go. It sits on the dashboard basically where the cup only prevents it from moving but doesn't hold any weight.
I tried once to put it besides the rearview mirror, with the power cord around the mirror's mount. One big bump in the road and the cup let go, the GPS dangled for a second on the mirror mount, held by the power cord, just long enough so it could get enough momentum the power cord disconnected and I ended up with the GPS in my face. Not very fun in fast heavy traffic. I had no time to react, I just turned my head and got it on the side. If anything would have happened outside the car at that moment, I would have had an accident. That was the end of the high-mounted GPS.
I think California repelled the law about mounting stuff to windshield, in order to allow GPS. I read this on this forum I think. But if I recall, the law stipulates it must be mounted as low as possible and not obstruct visibility.