How do you tell which way the factory is orientated? I have a hard time with this when planning construction. Is there a way to rotate factories and may I suggest that there be a visual indicator on the ghost image on which way the roll off is? Or a new model factory with only one ramp, since only one is used anyway.
A unit will rolloff in whichever direction the factory's rally point was pointed when construction of that unit started. So, if you set the rally point to the left side while constructing the factory, the first unit that it builds will rolloff in that direction.
Units will go off the factory in either direction depending on blockage or waypoint. So it doesn't really matter at all. It's better to have two rolloffs, since they'll go off in either direction.
Sometimes with a line of factories you don't want to set up a rally point as you'd need to select them all and set one; I often just leave them with set roll of direction but it's annoying when some units end up going the opposite way of the others...
The green stacks on the factories represent the front of each one. When looking at the factories from the front side the default rolloff is to the left. Air factories rolloff to the front by default as seen in this picture. this is how they rally when no rally point has been set.