Yeah, if some of them are *not* sleeping on the roosts (other than silkies), you want to figure out why and fix it.
Much more hygeinic on roosts, also safer from predators, and mostly it is just the basic nature of chickens to want to sit on something elevated so if they are not doing it then something is wrong.
I agree that providing a foot of roost per chicken is a good situation -- it allows them a little leeway to sort out social issues. Even more is even better, though unless you have major incompatability in your flock the extra may not get used much.
Mind, you do often have to train them to roost -- go in there at (or just after) dusk, and pick up any stragglers and PUT them on the roost. They will 95% of the time stay there, and after a day or few days of waking up there, they'll start putting themselves to roost.
Good luck, have fun,
Pat