Good Morning, Frank,
Plastic reels, by nature of their construction, have more crevices which can contain chemical residue. SS reels do not have that problem. If you drop your processed film into a film washer, the reels get washed at the same time as the film. If you use Photo-flo after washing, putting the empty reels back into the film washer for a minute or so takes care of cleaning them. Just to be sure, I'll usually finish by spraying some hot water on the empty reels before putting them aside to dry. The same general approach can be used with plastic reels, but a bit more time and care is needed to be sure no trace of Photo-flo remains.