Maybe worth doing, as said before most people do the ME combined with CPL or IR but doing the multi engine rating before either I suppose would get you ahead of the game if you are going to do your CPL and IR in a Seneca. As it gives you more time to concentrate on your flying rather than learning new systems and checklists but then it doesn't really take long to get used to so I think if you did the rating before you would probably end up spending more.
Hardest thing for me to get used to on the Seneca was taxying the bloody thing hehe