There is a physiological basis for the "magic" 2mm exit pupil.
A 2mm exit pupil most closely matches the area of highest resolution in your eye and gives you good detail for planetary, lunar, and globular cluster observing. The sharpness of those details is likewise improved by a 2mm exit pupil, as a smaller exit pupil minimizes astigmatism at the edges of your dark-adapted eye. Also, the visibilty of small galaxies and planetary nebulas is often enhanced by the darkening of the sky background with a 2mm exit pupil.
Mel Bartels discusses observing strategies based on various exit pupil ranges at http://www.bbastrode...com/visual.html
What magnifications should be used? I favor three strategies both based on exit pupil (the eyepiece's focal length in mm divided by the telescope's overall focal ratio [e.g., 24mm eyepiece on a F/6 scope produces a 4mm exit pupil]):
The first is based on Richard Berry's advice. Arrange your eyepieces so that they give exit pupils as following:
5-7mm Richest Field observing
3-5mm best deep sky observing
1-2mm best detailed observing (globulars, planetaries, lunar and planetary)
The second is based on Stephen O'Meara's comments (e.g., his Herschel 400 Observing Guide). He uses modest aperture (4 inches [10cm]) at low, medium and high powers. He takes his time studying the object carefully at each power. His low, medium and high exit pupils are:
If you are wondering who to look to for observing advice, pay attention to the top observers who use smaller scopes, like O'Meara.
The third is a strategy that I've developed in response to the super wide angle eyepieces available today. It allows me to see large scale objects otherwise too big for a given scope. I call this strategy “framing” or “composing” the view where the object is magnified to fill the eyepiece’s field of view as much as possible with a nice border around it for contrast. Increasing the apparent object size beyond this 'cut-off' results in a less pleasing more difficult view. Here, the widest possible field of view is important, even at the cost of more glass for the light to pass through. In this approach, I smoothly decrement the exit pupil. I use a set of exit pupils as follows (note that the typical set of eyepieces does not fit nicely):
5-6mm for largest scale objects
3-4mm for medium scale objects
1-2mm for small scale objects
Finally, poor seeing conditions especially with larger apertures will limit magnifications to 200-300x or 2-3mm exit pupil.
Since the majority of deep-sky objects that are observable in amateur telescopes are rather small in apparent size, using a smaller exit pupil makes sense. My experience has been that an exit pupil range of 1.5 to 2.5mm works very well in most cases.