[thaumasiotes]

I've seen a few theories in various places:

- Match the type of accidentals used by the key signature.

- Use sharps when the melody is moving up, and flats when it's moving down.

- Use whatever accidentals give you a visually pleasing spacing of notes in "thirds" as represented on the staff. (As advocated here.)

- Use the accidentals that are physically compatible with other notes in the chord. (Only a concern for certain instruments, but e.g. I understand that the pedals on a harp affect many strings at once, so playing a particular combination of notes may require a clever approach to the pedals, indicated by what might otherwise look like an unnatural set of accidentals. This wouldn't apply to a piano, where G# is just a separate key from G, not a modification of the G key.) I think this is what you're going for?

[skybrian]

Another example would be the bass side of an accordion where Db and C# imply different buttons (though not different sounds - there are duplicates) and they’re quite far away from each other.

[coliveira]

The accident you use really depends on the context. If you're talking about chords, then you should follow the "stack of thirds" explanation that I gave above, it is how chord theory works. However, the other rules you mention also can be used depending on your situation. In a melody, you normally want to add accidents only when needed.