I can't speak for the average cube, but since I made the tactical cube I can vouch for it's 360 degree weapon arc. In fact, it has at least 9 weapon points facing in each direction not counting the big cannons that protrude from the hull. I might take a look at the others at some point, especially to even out DPS per weapon point compared to what the tactical cube currently has.
In general, all ships will face forward if they have weapons in that arc. If weapons are primarily for broadside attacks then they tend to turn a little bit. The best bet for your cubes is to take them into the heat of battle and let them blast away at ships all around them. The tactical cube does spin a bit as it fires, but I think that's because I've got weapon points on each edges of the cube.
I'm sure you did see cubes flying around and possibly banging into each other. If you look at capital ships in unmodded sins you'll see that they move around as well. But, they're much slower and much smaller, so the bumping around is a non-issue. As I said, Stress has solved this issue with the federation ships in the diplomacy foundation by reducing all model sizes and ship speeds by half. We'll have to wait and see what kind of effect it has on the cubes, but I can definitely guarantee that it'll be an improvement.