I think the reason people wait is because when you add the KMS and the Sorbate, it can change the flavor for a coulpe of days. I usually know how much sweetener I want to add and just add it when I stabilize it.
Not only that, but if for any reason the sweetener wasn't sanitary, the KMS will take care of it. You also don't have to mess with it again until bottling.
Hope this helps.