I did my IMC prior to starting the CPL and IR, and I think it was good preparation, and value for money.
If you're renting from a club then the difference in cost per hour from flying solo or with an instructor isn't that great - and if you have been flying for a while since your PPL it is good to get everything polished up by having the instructor there watching and pointing little errors or sloppy habbits.
You do some basic instrument work in the CPL, much the same as from the IMC, so it will prepare you well. It will also prepare you for the IR - and maybe save you some money too - learning the basics on a C172 or PA28 will be less costly than doing it in a Seneca for example during the IR.
Once you have the IR and CPL issued then it makes little difference that you did the IMC, and when you do the courses you won't know if the IMC saved you time and money, but I would say if you want to do it then why not?