Depending on the where the sun is and how high you are flying.
The open windows are like searchlights. Its super glaring becasue you are above cloud levels and its extremely bright. Not to mention its bad for your eyes being so bright.

Things to consider

1. LCD screen viewing of the people around you area is affected badly.
2. Perons next to you is reading a book or is not sleeping

When I feel bored and want to look out, I do so at the exit doors at the rear of the aircraft. Not all of them have windows but some of them do and you can enjoy the view as much as you like.

No one will say anything if you open the windows for a snap shot or to view outside. But if you want to keep the windows open, make sure its not super glaring, if not the Fas wont let you keep it open.