Not required at all for PPL instructing.
The rules for IRI are about to change so you'll need much more experience before becoming an instrument instructor - there is a lot of talk about where they will find IRI's from in the future with the pre-reqs they're laying down.
For bog standard PPL/night/aeros instruction the IR is a nice to have, but not a necessity in any way. It may open up your options a bit in terms of some of the safety pilot stuff that pops up - but I wouldn't hold my breath.
My recommendation - CPL, FI, experience, IR, experience, IRI. May take a few years....
HTH
DD