First question, not really but if running a diesel it helps minimize vibration as most of the rotational forces are absorbed throught the engine mounts and equally distributed to the morth and south frame cross members.
Second question, A horizontal axis positive or negative degree change can cause problems. For instance, excessive u joints wear, excessive shaft vibrations, a pull to the left maybe, and in my opinion if you have the drivetrain and the shaft connected on an angle, the torque transferred through the shaft to the diff will be transferred unproportionately, thus causing more strain on the shaft, and possibly damaging the diff.
Third Question, Vertical tilts are very much normal with most engine fittings, because due to body and chassis structure some don't fit in straight, and there is good reason the tilt reduces vibration, but a horizontal tilt would be the same as the answer for the second question, the best way is to get a balancing tool, one used in construction, a small tiny ruler with a bubble in it, i don't know whats it called but anyways, that could tell you only whats what when the engine is in. If you wan't to check it before the engine goes in, map out your engine, measure each side, measure the mounts, their height, distance between the firewall and the brush guard, and if there is a difference you will see it coming.
Fourth question, a minor change in position is not problem, but a change in horizontal angle would be the same as the answer to second question.
I hope i'm right and this helps.