Personally I did a year as an FI before I sat the IR. A few points on the subject.
1) You present yourself to a CFI without an IR and they guy will think "Great, someone who won't naff off to the airlines with only 1 weeks notice". All CFI's know that instructors will eventually leave, but without an IR you are less likely to leave at a moments notice and hence will be more attractive prospect as a newbie FI(R).
2) Get the IMC for all the reasons stated above. It will give you more options and also increases your ability. Never a bad thing.
3) You will be happier in the job as well. Guys with IR's spend most of their day moaning about the number of PFO's they got and how matey across the apron has just got a 737 rating. Without the distraction of airline flying you can concentrate on instructing. The students appreciate that as well.
4) When you finally come to sit the IR your handling skills are usually so much higher than a 200 hour CPL chap(ette). Plus you have confidence on the radio and you have much more capacity whilst in the cockpit. I noticed this when I was buddied up with a 200 hour guy for my IR. From a handling point of view the guy was spot on, but once we left the sim and started bashing Glasgows ILS, the guy was totally maxed out and took him a little bit (5-6 hours @ £330) longer to gain the capacity.
5) Make sure you can get the time off to do the IR. It takes 6 weeks (including MEP) and you don't really want to be mixing your flying with instructing and training. I was forced to instruct on the weekends and train during the week. It was very hard on me to do it that way and would not recommend it.
6) If you get a job at a school which does IR training then you might get a reduction when you sit you course. Don't ask you don't get.