Handlebar angle
 

This question may have been answered here before, but I was unable to find it. I'm building a new MTB with a 2" riser bar and am wondering about it's angle relative to the bike. Should the bars be perpedicular to the ground, or raked back at a similar angle to the headtube or, does it not really matter much? I'm just concerned with potential slipping (forward/backward) if not installed properly.

Thanks.