Poser is designed for rendering smoothly curved surfaces such as flesh, especially female nude figures! It doesn't do quite so well when you want it to render a flat surface. For instance, if you download my 'two platforms' prop that you'll find on the Poser Forum Fun Stuff page, you'll find that it appears in Poser to be made of rounded logs. If you load it into Max or RDS5 on the other hand, you'll find that it appears to be made of flat planks.
I've had some discussions with Metacreations on this subject and there are apparently two ways around the problem. The first approach is to provide a chamfer on every edge or corner of your model. This chamfer should be as small as possible, just one or two units in width. My first attempt with this method wasn't very successful, possibly because I made the chamfer too large. I've since had another go using a chamferbox and the smallest chamfer that Max would allow, 0.01. This worked fine and produced a nice flat sided box. I wanted this to become a wall with a door opening, but after cutting the opening by boolean subtraction the wall developed the characteristic Poser shading that ruined it. So now I need to find a way of adding a chamfer to the edges of the door opening. My max skills don't run to that yet, but I'll keep trying. The second approach is to use a lot of fine detail in your model. I originally made planks for my platform prop using just eight vertices, ie just the corner points. If instead you use a few hundred points by using multiple segments then Poser will render the object much more successfully. If you export the cube prop that comes as standard with Poser 3 and examine it in Max or RDS5 you'll get the idea. The downside of this approach is that file sizes can increase significantly. So, as with most things it's a trade off, but in general I think it's worth it. So if it's just a flat surface then apply a fine chamfer. If it's got an opening in it then you need to use lots of vertices.
As mentioned on the utilities page, Poser uses a very tiny scale compared with most other 3D packages. My Objaction utility takes away much of the heartache associated with transferring .obj files between Poser and other packages, but there are still problems when first creating a prop. My approach is to make a series of spheres of different sizes so that when imported into Poser they are approximately the size of a marble, a golf ball, a tennis ball, a football and whatever you call a ball about ten times as large as a football. You save these either as individual files or as a group file and by merging one or more of these with your new prop you can accurately gauge the size of the end product in Poser. Then delete the sphere(s) before saving your prop. This technique is nice and simple and you don't have to worry about units or dimensions.