Vans addresses it in the plans. I put it between the stiffener and the skin, which I've seen people do. The concern I had with putting a blob between the two stiffeners is it just falling off after time.
I also think vans addresses the fact that it's not really necessary and a lot of the cracking was a result of over driven rivets. At the end of the day, don't get too wrapped up about it, it's not a major issue. If it make you feel better then do it, if it doesn't make you feel better then don't.