I keep them closed and then open them when the sun goes around to another side of the house. My husband doesnt even like me doing this. He says that the curtains keep the heat from the hot glass coming into the room. Not sure if that is correct or not. However, I have had two visitors to my home say to me that I sould have the blinds and curtains open as ut doesn't display the garden as well as they think it should be. I can't get over Australians at times. Just the fading of the rugs and bedding and even furniture in summer makes it obvious to me that you do NEED to do something with the curtains. I have been told by Europeans that, tho they absolutely love wallpaper, that they wouldn't even consider buying or using it here in Aust. because of the very strong UV light.
It depends on the room and the side of the house where the windows are!! I love to be able to see my lush garden outside the big glass sliding doors to my TV room,but I do not mind having the bedroom blinds shut as the dimmer light is definitely cooler in that room!