I used to do the same until someone broke in and stole it when it was parked in my drive. Unfortunately it does happen, although I can see why you'd be happy to take the risk. The annoying thing isn't the satnav itself being stolen, it's the damage they do to your car in the process.
I have mine in the bottom right of the windscreen and it's almost impossible too see it unless you are actively looking for it. Spends 72 hours a week sat in a dark supermarket car park in all sorts of shady estates.