Keyboard Cover/Skin on MacBook

Does a keyboard cover on a Mac affect it's ability to vent properly? I saw a comment from Apple that you should take it off before closing it because of lack of space between the screen and keyboard, however there is also rumors that it can cause your Mac to overheat; is this true?

I would like to use the cover that I bought for my Mac just to prevent dust, crumbs, and other debris from getting in between the keys, but I don't want it to hinder the venting process. Thanks!