Key Signature question

[sinfish]

Are there any hard rules to Key signatures when it comes to noting accidentals?

If the Key of the song is G# major/Ab major what key signature is used and why?

Is it what ever has the least number of accidentals?
Is it determined by the ascending / descending movement of the music?
Is it determined by the feel of the music?

Thanks for your help

 

[degenaro]

For all intends and purposes in your usage go with the least amount of accidentals...so...G-D-A-E-B-F# get sharps and F-Bb-Eb-Ab-Db get flats.

 

[sinfish]

thanks!