My macbook runs fine, but I have noticed it overheats sometimes, and when I looked at the back vents closely with a flashlight I noticed there was a little piece of tissue or something stuck in there and dust. Is it safe to do it with your computer shut off and spray a little air into the vents at the back of the computer? I hear mixed stuff about this, although a guy at Futureshop told me you can just use compressed air to clean out the vents. I don't want to take apart the whole comp to clean the fan, I'd just like to clean the vents. Safe or should I not do it?