Some good debate here....I went ahead and ordered some insulation pads for the vents. Thanks!
Air is sucked into an air conditioner, cooled, and then forced back out. If a vent is open, it sucks that air in as well -- so if you leave the vents open, you are now cooling the world . . .
This is not really debatable, IMHO . . .
Yes, seal every possible vent, crack, hole, or wherever else outside air might be sucked in. For everyone you miss, the efficiency of your AC will be decreased.