As James said, there are a ton of variables to consider on this to be truly accurate, but I suspect, given that your average window ac unit draws 1000 watts per hour, versus a 25 watt CFL that if keeping the drapes closed meant even just one hour less of cooling, it would probably be worth it. That said, you could always close the drapes just on the south facing windows and the east or west depending on time of day and that would probably give you a lot of the advantages while still bringing in some natural light.