Hi there!
How old is the child? A newborn's head needs not to slump (that's why infant seats are generally around 45 degrees). That is about making sure the baby's airway is open.
As soon as a baby has good head control, we don't worry about head slump. Here's why for rear facing: in a rear facing car seat, in the vast majority of accidents, there will be either a forward collision, or braking at the least. The child's head and spine will move into the shell of the seat (toward the front of the car), and the shell of the seat will absorb the crash forces. That motion is why rear facing is so much safer for small children with big heads and still-developing spines.
Maybe a crash test video of a rear facing car seat can explain the dynamics better than I can--here's a very short one with a dummy where you can see the motion toward the front of the car:
and here's an animated one arguing for the safety of rear facing versus forward facing. It has narration but also shows the crash dynamics clearly:
[ame="[URL]http://www.youtube.com/watch?v=sssIsceKd6U&feature=player_embedded"]Rear[/URL] facing vs forward facing position in the car - YouTube[/ame]
Many convertible seats can be installed at a range of angles. If you post what seat you have and what car it is in, it is possible we can help you make it more reclined. But if the kid isn't concerned, I wouldn't be: they're just more flexible than we are and can sleep in all sorts of positions that would make my neck ache for days.