Since I'm not the style guy, I've been wondering lately if there's any real distinction between RIS and a plain ol' Imperial Stout. I see most commercial brewers just call them Imperial, without the Russian thing.
Regardless, I typically use WY1084 in my stout. I tend to call it a strong stout, not imperial, but it starts at 1.088 or thereabouts so it fits the bill. I have no particular reason that I use 1084 except that's what I started using to make stouts back in the early 90s. It attenuates well and I'm happy with the end result.
I've also made this same beer with US05 and with WY1968 (ESB) and of course with a Belgian strain one time when I pitched the wrong starter. I don't recommend going the Belgian route, though it produced a decent beer, if not what I was planning on.
FWIW, I haven't found too much difference between US05, 1084 and 1968 in a big stout. I think this is simply because there are so many other flavors going on that the yeast takes a back seat. As long as it gets you the attenuation you're looking for, IMO, there are a variety of yeasts you can use with good results.