I’ve seen this question posed on numerous sites and seen a lot of responses, but I wanted to take a crack at answering why spaceships always seem to meet right-side up in sci-fi stories. The most extreme example of this is Star Trek, where the ships always are shown on a parallel plane, but it happens in Star Wars, Firefly, pretty much every sci-fi space epic.
The obvious answer is because it would look completely stupid if ships showed up at various angles to one another:
But there is another reason for this consistent plane! First off, we all know that the Milky Way galaxy, and pretty much all other galaxies we’ve ever observed, exists as a large spinning disc. Our galaxy has several arms, but overall it’s shaped like a giant Frisbee. This means there is a galactic elliptical plane, a “top” and “bottom” to the galaxy. Which end is which can be up for debate, and the galaxy is pretty thick in places, but there’s still an up and down, or at least a galactic north and south.
By the same token, star systems have an elliptical plane, usually determined by the rotation of the main star or stars. The solar system is like that, with one oddball that orbits at a weird angle, but the rest of the planets all lining up like we were on a big vinyl record. So here too there is an “up” and a “down” if we define the solar elliptical as the mid-point. Ships in almost every sci-fi show align themselves with these ellipticals, but why?
Well simply put, for the same reason you and I stay within the traffic lanes on the road. It makes it safer for interstellar travel and it makes it a lot easier when running into another ship if everyone is aligning on the elliptical, even if their “Z” axis is higher or lower. Picking which was is up is just a 50-50 chance.