I instructed very successfully for two years without ever holding an IR. I did the basic IMC rating and once I had 100 hours instructing upgraded to an IRI instructor which allowed me to teach the IMC rating. Gained loads of great IMC flying which made my IR a little more straight forwards.
So you don't need the IR but I would certainly recommend at least having the IMC rating as you will find you self in situations where having an IMC rating really helps.
