Well, I would say you should definitely have your basic skills in check before you leap into an MEP. You will be a lot busier for sure. If you have always had strong basic skills, then by all means go out and do a bit of SEP airwork to refresh yourself then go do the ME/IR. What opinion schools will take of this remains to be seen.
I did my ME/IR before my CPL and I would say it was questionnable as to whether it helped or not for my CPL. One thing it does do is make you fixate on what's going on inside the aircraft more than out the window, which isn't good for the CPL skills test - e.g. slow flight...it took me a while to re-learn the idea of setting power and pitch attitude with visual reference to stuff outside the airplane - I was glued to my Altimeter, DG, and ASI to start with. You basically end up trying to conduct a VFR flight more by reference to your primary instruments than anything else.
I would, with hindsight, say go and do the CPL first. The ME/IR is a large learning curve. Build on the basic skills in the CPL then jump into the twin...