My roommates and I have been debating this. I know it's good to have the blinds open on sunny days during the winter, but I don't think they should be open on days when it's below 50 and the sun is not shining. I think blinds can act kind of like a vent on a register. If you want the outside temp to come inside then you should have them open, if not then you should have them closed. Sure it won't stop all the hot/cold temperature from coming in if they're closed, but it seems to me that it would help.