My desktop has it at the top, as did the PC I had before it a few years ago. I've seen PCs with it at the bottom, but I've never owned one, so hearing that they're always at the bottom is weird to me.
But then your desktop will be upside down on your monitor, as obviously your harddrives will be upside down. Do you have any idea how hard it will be to watch upside down porn?
It may be different with brand specific cases for prebuilds like Dell and Alienware (which is Dell too by the way), but it's never recommended to buy those.