Ooooh,
Interesting question and believe it or not there is no definative answer.
With the latter day machines, i just put the spirit level across the PF and level off that way - do it top and bottom. As for angle - thats entirely up to you, floaty balls are a pain but so are super whizz down the gullet balls - the balance is up to you.
With some of the older 50's 60's games you balance the machine to game play, depending on the design there may be a preference for all the points and game play to one side, or a drain on one side. Slightly off angle balances this out. Some of the rollover shots can be tricky especially if you are using the correct size balls for these as the gap is tight. You dont want the ball floating around these, but you also dont want it smashing through them.
With the woodrails, i always balance off with the spirit level then fine adjust to game play after that
