Gday Ken
There is no true "convention" on this ( as far as i can see )
Some authors of celestial calculations use Sth as zero ( ie Meeus/Trueblood and Genet ) and others use Nth is zero ( ie Duffet Smith ) .
The arrangement of the calculations to convert RA/DEC to AltAz etc then depend on the convention chosen.
( Meeus notes that the Nth vs Sth convention was different between astronomers and navigators
)
I know Meade use the Nth is zero convention, probably so they can match it to the "Level/Nth" start position, so to simplify the internal calcs, Nth is still zero, irrespective of hemisphere.
Andrew