Right foot down most often though I ride with half-clips and platforms so I use either foot sometimes.

• I mount the bike from the left side and it seems more natural to plant my right foot and then position my left on the pedal
• If I "cowboy mount" it's my left foot starting the stroke
• I coast left-forward so it's more natural to start pedaling on the left.

Road slope is meaningless to me but if it was a concern, I think I'd rather put the down-side foot down. Less chance of accidentally leaning the bike the wrong way that way.