It can't hurt to try. I wish I had bought a cal kit when I purchased my FA-VA5, but most of my measurements are at HF, so I convinced myself that the basic 50 ohm load was good enough.
Have you considered building a few calibration loads, just for a 'sanity check'? I used a BNC-to-SMA adapter, but you can also use a BNC panel mount...
I've been pleased with the unit's performance measuring components at HF. From my perspective, it's more intuitive than a nanoVNA. Now I want to turn my hand to measuring crystal ESR or Q, as some filters I've put together for my NC-40A projects are very odd...
va3rr