Some milling spindles actually give the tool a 45 degree inclination, that helps you prevent kinematic singularities (e.g. when the axes of A04 and A06 coincide) as the arm will more often be tilted (compare e.g. to the Elte spindle in the library). However, from our experience this may again cause reachability problems when processing elements vertically.
As often with the robot there is no "best" solution, but a trade-off!
Best,
Johannes