One of the things I see from other people's models is how straight all of the lines are in the mesh. And yet when I look at my own models in Blender they appear to be mess. Of course, when I do import them into Poser and view it in wireframe mode, they do appear to be straight. I'm assuming Poser is straightening the lines in the mesh for me, but I wonder if this might not account for some of the artifacts I sometimes see in the mesh when it renders.

Is there a way to ensure that the lines stay straight when I need them to?

Can you post screenshot of an example? Or a link to a .Blend file where you have the issue so people can take a look and offer more specific advice? It's hard to imagine what it might be without seeing an example. All I can think of is maybe you have a Subdivision modifier on it in Blender which is not exporting, so when you import into Poser it is no longer rounding the mesh.

I'd be really surprised if Poser is adjusting the mesh in any way, although I haven't used it since Poser 4, so what do I know?

I'm with MDO2010 on this one. Can't figure out what you mean by straight lines in the mesh. Do you mean edges? Edges should be perfectly straight from vertex to vertex no matter what, so if you are seeing wavy or broken edges in your mesh in Blender, then that is a sign of some kind of display issue with your graphics card or driver. I've heard of that kind of thing happening, but not very often. I'm interested to see an example though, you got me curious.

If you want to straighten out a row of vertex? Select the vertex you want to level out and tab : S (for scale), X,Y or Z for the plane you want them straight in, and then ZERO on your numerical keyboard.

On your second remark : Poser does not straighten out. It loads "as is" in the obj file.