Here's an article in Street Rodder concerning angles. Hope it helps.
http://www.streetrodderweb.com/tech/0203sr_driving/
And another informative page:
http://4crawler.cruiserpages.com/4x4...line-101.shtml
Looks like close to parallel is the best. I'd say if you don't expect any pinion shaft movement (the diff case or mount might move under load a small amount), then set it up 2 deg to be close to parallel but leave some room for movement.
The pic courtesy of
www.madxj.com