Actually, I think the models look quite nice. I'm not a modeler myself so I can't reasonably critique them in terms of their construction, but the end results are very good in my view.
I don't think that you've done anything necessarily "wrong" in your modeling, either. The quality settings in non-RTO rendering are there to help strike a balance between render time and quality. Non-RTO rendering just isn't as fast at rendering high quality objects. If you *must* use non-RTO and the issues only occur in a population, then just use high population quality. If the issue occurs in single objects, I don't know if making it into a population and reducing it to a single instance through population controls, then setting it to high quality would help. I think the less complex your objects, the less likely problems are to occur, and perhaps if you add more detail in through displacement (with RTO *off*, of course) and less through actual geometry, that also would help, but I'm not sure of that.
- Oshyan