I found this, but it doesn't say where the safest position is on the windscreen.

The ease of fitting is clearly an advantage since there's no (or very little) fitting cost involved. However, as well as giving a rather untidy appearance and being somewhat prone to detaching itself in strong sunlight, the typical glass-mounted arm makes a very visible target for thieves.

Portable navigation units have replaced the car stereo as the most desirable item for the opportunist thief. Unlike a CD player they are neither difficult to remove or protected by a security code or removable panel.

Even if you religiously remove the device from the arm and carry it with you (not always convenient as some are more portable than others in terms of their shape) the hawk-eyed thief who sees the arm on the windscreen may well break in anyway, assuming that the device will be in the glove-box. Our advice is to always remove the arm from the windscreen when you park.