Like Brian said, no easy definitive answer for every situation. And the Stereophonic Zoom is also a good place to start. But another, maybe easier starting point from which to think about it:
DIN (90° 20cm) is often a good place to start if you are in a location where:
1) The sound source is reasonably spread out in front of you, but not encircling you (or even making a 180° arc in front of you).
2) The balance of direct and reflected sound is pleasing.
3) The overall frequency response is pleasing - not too bassy, not too thin.
Narrowing the angle between the mics and/or moving them closer together will:
1) shrink the stereo image on playback
Narrowing the angle between mics will also:
2) emphasize direct over reflected sound
Moving the mics closer together will (in addition to #1):
3) tend to reduce the bass response
So, if you are right on top of the sound source (eg onstage or stagelip) you might want to consider XY-90 rather than DIN because now the sound source is spread very wide in front of you and you want to shrink things down a bit for playback. Or if the bass is booming, XY might be a good choice.
Conversely, if you are way far back in the venue XY-90° might not be a very good choice, as it will shrink the direct source soundstage down even more and emphasize the feeling of being distant. But even DIN 90 might have the mics picking up too much reverberant sound, you might want to have the mics making an even narrower angle. This will also shrink your playback image, so you might want to move the mics a bit more than 20cm apart to compensate. But that could accentuate the bass, maybe too much. Ultimately when you are taping from a less than ideal location you will have to make compromises. IMO the stereo image is pretty much shot from way in back anyway so you might as well stick with DINish spacing unless you want to emphasize the bass. And if it's really boomy, might as well run XY.
Outdoors, excess reverberant sound and excess bass are much less of a concern. So if you want to create a more spacious image (e.g., if you're a little farther from the stage than you might like), you might consider angling the mics out a bit more or spacing them a little further apart. Like maybe 90° at 30 cm.