I've never had a problem. Shading rate slows the final render, but so far, has never choked Poser for me, nor generated out of memory messages. Probably because I only use a shading rate of 0.05 or lower on things that need it. That's the beauty of using the shading rate. You don't have to render everything in the darn image at that rate, only what you want to.

It is kind of funny to watch it render, though. It goes really slow over the hair (or whatever), then speeds up and zooms over the rest of the image.