The shock is less effective when mounted at an angle. If the shock is mounted 90 deg from vertical then the shock won't have any effect at all in damping oscillations. It's a cosine function. 30 deg reduces damping by 13%.
You're also slowing the shock piston down when mounting at an angle. Slower piston speed also means less damping.
But all this may already be calculated into the shock you're getting.
If the choice is between mounting towards front, or towards back, I would choose front. Mounting inwards, towards the center of the chassis is a bad idea.