CPM (Adbrite) – Does ad placement matter?

I know this may be a stupid question, since CPM is "cost per impression", but does ad placement matter on CPM ads?
I’ve only ever used CPC (Adsense), but I’m thinking about signing up for Adbrite. Whats stopping me from just throwing some ads way at the bottom of the page where no one will see them anyway?
It just doesn’t make much sense to me. Please let me know, thanks!

See original post by jayvanbuiten