Section 6 (page 73) of the FT300 manual states that every 6 months the user should do an inspection of the sensor as part of a routine maintenance schedule.  Is this simply a visible inspection for wear or damage, or something more in-depth?  Is there some period of time where Robotiq recommends sending the unit back to the factory for recalibration?  

@joel_moser visual inspection yes, no parts can wear on the FT-300. However, you will see a line at the center of the FT-300, that line is not sealed and fluid could leak inside the FT sensor. Make sure it is clean, if it is exposed to fluid you should seal it with a rubber band for example.

Factory calibration is made once and there is no reason to re-do it. You could do the software calibration, this one can drift over time if the mounting screws torque changed for example.