What's odd about this topic is that there does seem to be SOME directionality to ramps.
The entrance to my fortress goes along a straight shaft, down a ramp, along a hallway and around a bend, and down another ramp. That middle level does not have ANYTHING beyond the ramps, only the floor between them - the ramps are up against walls on either end.
When I dug behind the end of the corridor on the uppermost level to put a ballista post, and then tried to put fortifications, I found something odd.
If I finished a row of fortifications near the ballista, I could not put in another row of fortifications nearer the ramp because it had no path.
So, for some reason or other, than particular ramp DOES only work in one direction. I'm assuming that there has to be a sensible path from the bottom to the top. If the south side of the ramp's lower level is against a wall, you can't come from the north side of the lower level and go out the north side of the upper level.
Isolated ramps can be seen as incomplete. Until you put a wall on one side, they don't really go anywhere. Finishing the ramp is in reality a part of building that adjacent wall(or the floor on the level above).
In other words, the dwarves are doing some of that work behind the scenes.
As for what happens when the ramp goes both ways, north and south to use my above example... I got nothing.