I think it's one of those things that can depend on the circumstances, e.g. field layout, opposition, number of players etc. Sometimes it's better to stand and shoot (better accuracy) then sprint to your primary once the middies are in to give the cover, sometimes to move (slowly) towards the barricade while shooting, and sometimes to have some backs doing each. E.g. in 7-man if you have one big central back coke can or similar and 2 very wide out back corner bunkers, you may wish to have the centre back standing & shooting before stepping in, while the corner guys shoot on the move. If you're going to push 2-3 fronts up onto the 50 on the break, you'll want to do things differently to when you are all going for a short break.
One thing you should very rarely see is the backs sprinting for their cover firing wildly in the rough direction of the opposition!
The moment I see that happening, I know I can concentrate on taking out the other team's fronts or mids without any worries that my own guys are going to get hit, and usually by the time the oppositions have made their positions even with them running, my own mids should be able to give me enough cover to get in myself.