I did a search and found some very vague stuff on this, but:

Today IAH to BHM on and COEx ERJ out of the bus terminal, so all the aircraft were sitting outside with the doors open baking in the sun, and the AC did little good...got onboard and in 12A, all the shades were down except the exit row, of course I pulled the shade to help with the cooling, and I've known before that it's (according to the FA's) CO's policy to keep the exit row shades open during takeoff and landing.

The question is, 1) is there really a policy for this, CO, FAA or otherwise? 2) is it against policy to close the shade once airborne? (as the FA told me today)

I know the drill on why the shade theoretically should be open in case of an emergency, etc, need to be able to see if there are any hazards before popping open the exit door, but I would simply open the shade if that was the scenario.