Well I'm not to sure on buses but on a light vehicle the angle of the engine and trans should match the angle of the rear end. 5% tilt on engine should be matched by an equal tilt from the rear end. As I understand it you'll get weird vibrations and eat u-joints for lunch.
my understanding as well. U-joints are not constant rotational velocity, they surge up and down as they rotate. The matching input and output angles make the changes cancel out and eliminates vibration.