You do not HAVE to?....
Actually, you are required by law to comply with ANY reasonable request by the crew onboard your flight. So unless they start asking you to jump up and down on the spot, rub your stomach and pat your head then i'm afraid that window blind does HAVE to go down....
However most crew will not make an issue of it.
Please remember crew don't want narky passengers they don't ask you to do things just to annoy you, but rather to help you. I have operated flights both leaving the blinds up and also requesting all blinds down and the passenger experience on the latter is far superior more rest is available for those that want it, those that don't have a reading light and IFE and with the high probability of cloud cover there is sweet FA to see down there anyway.