A couple of hints: Those curves (chamfers) between the risers really don't need to be so curvy unless you're going to zoom in and show an extreme closeup of those particular surfaces. Poser just loves to round things off, which is a problem in other circumstances but a virtue here. Probably a three- or four-slice curve will do just fine, which will give you much more controllable tesselation on the side of each step. Also, if you build the side rails as separate pieces (which will end up as separate Groups in the result) you can avoid the tesselation on them entirely. They can be plain old quadrilaterals.

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