When you're in orbit, it's the orbital plane, when you approach a planet, it's the ecliptic plane, when you're in the galaxy, it's the galactic plane, when two spacecraft meet, they're going to settle for one coordinate system as well.
Where, exactly, is the "orbital plane" around Earth? Is it over the equator, while all highly "inclined" orbits are illegal? Uranus has an axial tilt of 97 degrees—should one orbit in the ecliptic, or based on the local rotation of the planet? For that matter, why should vessels in the same orbit have the same attitude?
Suppose another ship of the fleet is approaching from zenith or nadir, thus making its plane of reference along that line of flight. What's the proper etiquette in that situation? Should the ship reaching the rendezvous point second reorient (for no useful reason I can imagine) just to be polite, or should the first ship tumble around to make the arrival feel more welcome?
To paraphrase Doc Brown, "You're just not thinking three-dimensionally!"
Keep in mind that these are starships designed for crossing unimaginably vast distances. While delicate maneuvering around spacedock may be possible, why would ships meeting in deep space even get close enough for naked eye visibility? Their communication (including transporters) is effective without having to rub elbows.
Don't confuse navigational conventions with the attitude of the ship. Heck, every 3D artist knows there are global and local coordinates.