I had an IMC before both the CPL and IR, I would recommend it if you have the spare cash. It makes hour building easier and more fun as you can tackle cross countries with a slightly more relaxed view, of the weather for example, it also means that whilst you do need to do 10 hours instrument work toward a CPL, it means that this 10 hours can be used where you are weakest rather than trying to cover everything, and an IMC also stands you in good stead for the IR itself. You are already familiar with plates, NDB's / VOR's etc and have come across some of the radio chatter.