When I'm trying to alter one note, Tux always proposes a sharp (like A#), while sometimes I would prefer (or music theory requires) using a flat like Bb.
It's not supported yet to define flats or sharps.
Today the application watch at the key signature of the measure.
if it's sharp based, so it uses sharps..
if it's flat based, so it uses flats.
and it uses sharps as default if your keysignature is empty (C / Am )