I think you use whatever it takes to get the image you want. The end product is what matters, not how you did it. I find it hard to understand why some people, and there are some, think you should do all the work in one program. If you were putting a shelf up would you say you could only use a hammer? You would do a much better job if you used a drill and a screwdriver too. I did an image this week which had a room rendered in Ray Dream Studio, view through the window rendered in Vue, furniture rendered in Serif 3Dplus3, all put together in PSP 7 on layers, and cartoon characters and their conversation added on more layers in PSP. That seems perfectly valid to me. You use whatever does the bit you are doing best. And if postwork will improve a 3d image, go for it. I very rarely enter contests, though. Apart from the fact that I am not a contest level artist, they so often limit you to one program, and I find that very frustrating, when I know I could make this look better if I did a bit of it in another.