I don't think putting volts on a capacitor stops it drying out! I also have a unit in storage.
Storage of spares is a tricky subject. Failure in storage is a significant problem: data retention in non-volatile data areas (including programmable logic devices); migration/recrystallisation of conductive elements - particularly due to lead-free solder technology; seizing or deterioration of mechanical parts such as rubber belts, bearings, or other metal-to-metal contact areas (particularly dissimilar metals); general deterioration of electronic components.
In general though, it is less stressful to keep a spare in storage than to keep getting it out to check it over, although you might do this if your job was to monitor the serviceability of spares and repair or replace them if necessary. Under our circumstances, there is little to be gained by knowing that the spare isn't working in advance of needing it.