If you are using the custom Tx 8-Ch setup that bypasses the FS One transmitter settings (expos, dual rates, etc), then you need to recalibrate. If you change between different custom 8-Ch setups that you’ve created, then you need to recalibrate. It is necessary to recalibrate because changing between different custom 8-Ch setups inside FS One with corresponding model changes on your real transmitter can result in channels being reordered, different trims, channel throws (maximum deflections), etc all being different depending how you’ve setup your models on your real transmitter (your real computer radio).
If you are using the FS One transmitter setups and do not change settings on your real transmitter, you only need to calibrate once. That single calibration will work properly for all aircraft in FS One.