I don't have much experience with professional flash units but I do have lots of experience with electrolytic capacitors being an electronics hobbyist and a formerly active amateur radio operator. I used to love older equipment.
It is commonly said that unused, electrolytic capacitors dry up and die. I actually have NOT found this to be true.
They seem to degrade used or not and eventually die. When they are used right up to the capacity, they tend to have shorter life. When used in higher temperature, they seem to die sooner. Conservatively rated and used, they tend to last longer. Of course, being a consumer, we don't have a choice on design parameters....
I personally don't worry about this. Use them when I need them. When they die, I just replace them.