Depends where you are, and which IR.
The JAA one is a lot of effort (too much classroom time for an oldie like me), so after a little thought, I didn't bother. It also has an annual renewal flight test. A friend of mine did the old CAA IR, and gave it up after a couple of years.
The FAA one has a biennial review (it's part of the BFR you do anyway), and an instrument currency requirement. That works better for me, but others have different views.
If you do your FAA IR on a single (not a problem), you can convert it to a multi at the same time as you do your multi-engine rating, and for not a lot more effort. I would recommend doing the FAA IR on a single, unless you already fly a multi. YMMV.