I've got a Peugeot 405, and the mounting angle is pretty similar - about 30deg. It's been installed for about 6 months (around 10-15 thousand miles) at that angle with no problems. I think that's a lower risk than taking my player to bits!
I think the manual's recommendation is on the cautious side. However, if it does break your player, you've got no comeback! I believe that disk failures (which is the most likely eventual result of too much shock at the wrong angle) tend to give a bit of warning (noisy bearings etc), so you'll have the chance to recover your tunes and put them on a replacement drive.
If you've got stiffened "sport" suspension, or do a lot of offroad driving, then believe the manual; for road driving in a standard hatchback, then I think 30 degrees is not going to hurt.
_________________________
Toby Speight
030103016 (80GB Mk2a, blue)
030102806 (0GB Mk2a, blue)