The safest position for the car seat is where it fits the best. If it installs equally well in all positions, then middle is safest b/c it offers protection from side impace crashes. But the outboard (door) positions are not unsafe. A good installation is one with less than 1" of movement in any direction. Its important to pick the position where the seat installs best, and believe it or not, that often is not the middle. B/c lots of cars have humps in the middle of the back seat, or bucket seats. And you can not use LATCH in the middle in most vehicles, you must use the middle seatbelt. Seatbelts are not less safe than LATCH, so that's perfectly safe.
Also important, is to keep baby rear facign as long as possible. The old 20lbs/1 year rule is just that. OLD. It came about in the 1980's when our seats were only capable of rear facing to 20lbs and we didn't know any better. Now we definitely know better and all current convertible seats rear face to at least 30lbs, but its hard getting people to realize this.
Turning kids forward at 20lbs/1year is an outdated practice that could cost you your child's life!
1)A forward-facing child under 2 years old is 5 times more likely to be killed or seriously injured in a crash than a rear-facing child of the same age.
2)A child's vertabrae do not fully fuse until 3-6 years old, before then, she is at great risk for internal decapitation. The spinal column can stretch up to 2 inches in a crash BUT the spinal cord can only stretch up to 1/4 inch before it snaps and baby is gone.
3)Current research suggests that children under the age of two years are 75 percent less likely to die or be seriously injured when they are riding rear facing.
4)In a recent article from Injury Prevention, it was found that the odds of severe injury to forward facing children age 12-23 months old was 5.32 times higher than a rear facing child. (Car Safety Seats For Children: Rear Facing For Best Protection; Injury Prevention 2007; 13:398-402.)
It works this way: when you get in an accident and run into something, the car stop suddently, but everything and everyone in the car keeps moving in the direction the car was moving when it stopped, in most accidents, this is forward. So in an accident with a child in a forward facing seat, his head, the heaviest part of the body on babies and toddlers, flies forward very forcefully and easily snaps. If that same child is in a rear facing seat, his head tries to fly forward but is supported by the back of the rear facing seat, so there is no stress put on the child's neck and spine.
Check out this photo album exclusively of rear facing kids, many of them much older than 12 months: http://www.cpsafety.com/articles/RFAlbum.aspx There isn't a single documented case of a child breaking their legs b/c they were rear facing in an accident. There are, however, lots of cases where children have been killed and seriously injured where a rear facing seat would have protected them better. And most kids actually prefer to be rear facing b/c they can rest their feet on the vehicle seat back. When they are forward facing, their legs don't receive enough support and will frequently fall asleep.