Well, that depends on a lot of things. First of all, you generally want your nest to be lower than your roosts, because otherwise your hens might try roosting in or on the nest box and pooping there at night. Secondly, the heavier breeds of chickens can't fly as easily as the lighter or bantam breeds, so if your chickens are the big girls, you'll need to keep everything lower down for their comfort.
Finally, you need to consider what will work best in your particular coop set up, and think about what will make life easier for you when you collect eggs. Even though my bantams are excellent fliers (and I've seen them scoping out the six foot high roof of their coop as a possible nesting site), I use plastic bins for nesting boxes and set them right on the floor of the coop or the ground of the run. I like plastic because it's easy to clean and mites can't hide in crevices because there are none. I like the fact that I can move the boxes around, too. It's come in very handy now that I have a broody hen.