It is not OK for others to smudge/grease/smear/taint/foul or otherwise violate your screen. No matter how expensive the screen is or who owns it.
I used to be one of those until I forced myself to take a pencil everywhere for if I could not hold back the urge. This keeps the screen clean and makes pointing more accurate.
My advice: Don't feel bad about telling them to back off - they may not even know this is not done. And if they do it again, compare it to if they had to use a (sweaty) friend's keyboard or mobile phone. So, limiting possible exposure to this is in everyone's favour.
I used to be one of those until I forced myself to take a pencil everywhere for if I could not hold back the urge. This keeps the screen clean and makes pointing more accurate.
My advice: Don't feel bad about telling them to back off - they may not even know this is not done. And if they do it again, compare it to if they had to use a (sweaty) friend's keyboard or mobile phone. So, limiting possible exposure to this is in everyone's favour.