I will just say that most people, including my wife, seem to be in the habit of placing their devices down face first.
When questioned as to the why of that most people say because they don't want their screen to scratch or something to hit their screen and damage it. For my wife, it's also privacy and I suspect that's probably a larger reason for a number of people here.
In any case, I've never subscribed to this reasoning. I have an obession with getting dust on my hands and I can literally feel the dust and debris on many surfaces, including tables in restaurants. For me, placing my phone or device face down is inviting all that stuff to scratch my screen.
If anything is going to scratch on my device I would much prefer it be the back. So I set my devices down on their backs, preferably with something between them and the surface they are laying on.
To date, other than scratches I know I have put there myself because of my own idiocy I've only had one scratch that I am aware of. And it took hard light from the sun at a very specific angle to even see that it was there.
So, if people are complaining about scratches on their screens I would argue that this is a result of placing your phone face down on a surface. If you don't even bother to make sure the surface you set your device down on that way is clean before you do it, well I can't help you.