The reason I let my water sit is to balance out the alk, ph etc.. sometimes i have to add things to get what i want. It is proably one of those things that can be done either way...some say its fine to add and some say no. Kent says its okay to add, but to wait preferably. I only wait because i usually have to dose something or another. I find it more convenient to do it my way.
I don't actually use kent...well i do sometimes i have a bucket laying around...but i have found salt is salt. I'd use reefers best but my calcium reactor cracked and i am too cheap to get a new one, and the calcium is low in reeefers best on purpose.