Depends what kind of an AI you've got as to the nitty gritty details, but in my experience they all have some kind of rate-sensitive auto erection system (oo er!) that will, in relatively unaccelerated flight, slowly return the indicator to a gravity-sensed horizon. Therefore if you erect it to some other random horizon in the hover or cruise, it will simply wander back to a true-ish vertical after a while.
Also, if you erect an AI gyro to a false horizon, you will get what they call a 'pitch-roll' error, whereby the indication will vary depending on which way you're heading. Try it and see one day:
Put on 20 degrees of bank, for example, and erect the AI to that false horizon, and roll out on a heading of, say, East.
Then, when you turn onto North or South, your AI will go wings level, but will indicate 20 degrees nose up or down, depending on which way you turned. It all has to do with the gyro's rigidity in space - just imagine the gyro spinning away rigidly and the aircraft and gimbals rotating around it as your attitude and heading changes.
So basically, you might as well set it to 0 pitch wings level when you're in a level attitude (visually in flight, or on the ground or whatever) and then just remember the attitudes needed for given phases of flight with reference to that.
Hope that is of some help.