I'm sure others can explain it better and have better guidelines but remember that the A notes you are talking about that you can play prior to the 12th fret are actually three different A notes. Low A appears two ledger lines below the standard notation lines. Middle A appears in the second space between lines and high A is one ledger line above the standard notation lines.
So right there you have narrowed your possibilities beneath the 12th fret to 2 possibilities for each version of the A note (low, middle, high)
Is there a trick to knowing which one to use? If there is I don't know it because I am still a relative beginner. I would think the deciding factor would be the song itself and where it is taking you. If the song takes you mostly into the higher registers I would probably choose the A note on the higher numbered fret - songs mostly in the lower register I would look to stay near the lower frets. Beyond that I don't think there is a wrong or right answer.
I hope that made sense and that I did not embarrass myself with my first post. If I am completely off base I am sure those more knowledgeable then I will follow with a better answer.
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