If you want to get really technical.....the tweeters are the MOST important ones to have pointing directly at your ears, with the equal path lengths, etc.
Mids aren't quite as critical, but since most people use a coaxial speaker....the mids end up in the footwells also. My own setup had two mids in each door with the tweeters in the floor, and aiming straight up to the listening positions. It was a really nice, clean sound and a solid front image.
/forums/images/graemlins/thumb.gif