^^^ Not necessarily. I'm guessing the isos that you have in back are there to help bridge the gap between the MAs and eliminate sag in the middle of the couch frame.
I would always suggest you try it all the different ways and stick with the one you like best
but I realize that can be a lot of work.
You could also try I M I M I across the back and see if that's an improvement.
Based on my experience, I would guess you will prefer more isos over less, because adding more seems to decrease net spring constant (make things bouncier and lower the natural frequency).
I'll explain what I mean by that in a little more detail than before...
If the isos were linear springs, doubling the number of springs would double the spring constant (make things stiffer). However, they are not linear springs, and they become stiffer as they have to support more weight. So in this case, doubling the number of isos also reduces the amount of weight supported by each iso, which make things less stiff. So when we double the number of isos, we are increasing stiffness by having twice as many isos but also decreasing stiffness of each iso because they are supporting half the weight. That decrease seemed to be of a greater magnitude than the increase in my experience because things became more bouncy overall when I added more isos.