I'm likely going to have worlds wrap in GT2. So, a unit could pass through the left side and appear on the right.
Of course, a unit couldn't hit the top and pass through the bottom, since the top and bottom of worlds don't touch. (You can't just hop from the north to the south pole.) Conceptually, it's a bit trickier. Consider this image of the map as it's being wrapped onto an actual sphere:

Imagine it continuing to wrap until the two red dots touch. At that point, in fact,
all the dots would be the same point, at the very top of the world.
Imagine a unit heading north toward the left, green dot. Instead of popping out of the bottom of the map (which doesn't make sense), it would actually pop out heading south at the right, green dot. Even though it technically makes perfect sense, I imagine that might be confusing to a lot of users when working on a flat map. They'd sent troops north or south and they'd pop out heading the opposite direction exactly halfway to the left or right.
So, should I make the world round only in the horizontal direction and not let units pass vertically, or should I set it to work correctly and risk people being confused?