Best to try and keep propshaft flanges parallel. When uj`s are phased/indexed + running parellel there is in theory no fore and aft motion on the slider into the gearbox . Get it wrong and you could get a mm or two of this motion on every turn of the prop. Imagine that spline going in and out at engine speed + overdrive. Its not hard to check, straight edge under the chassis, measure to crank centerline and the same at gearbox, then its just checking your running true down the centerline of the car.
3 degrees seem to be the norm.But parallel is important.
Bob
http://www.driveshaftshop.com/media//angle.jpg