Recently joined the team and did some screens since.
I also believe that a watermark is no problem, since the screens are there to give a better impression of the game. If someone likes to use screens for any reason (not included money-making) he/she could kindly ask.
The point about selling screens or using them to sell things is the reason to use watermarks.
Anyway it's not so hard to make screens yourself and then use them for whatever you like