The solution comes from pumpkins.

Open your F3 menu: when you place a pumpkin on the ground, it's facing you if your orientation is between -45° and +45°. At +45.0001° the pumpkin is rotated on way, at -45.0001° it's rotated the other. Good.

Now for some reason pistons and droppers don't follow that rule for the vertical angle… Here is how they should behave, and here is an album of inconsistencies due to the current system.