Hmm the position doesnt really matter because from what I have found out when you have 3 assaulters then lvl 8 skill doesnt work properly and gives additional round randomly to your other assaulters regardles of their position. I have main in mid and sexy yoruichi on top so she should receive additional round from my main 8th skill but my renji who is at the bottom gets the attack quite often even though he shouldnt have.
What matters more is the speed adjustment..
First kisuke then dark rukia then grimmjow because he reduces enemy defense then main and then sexy yoruichi so she can replenish some fury which gives you higher chance for other members of your team to skill next round as well.