Generally, Macs only hibernate (which means they store the existing system state to disk and then power off) when the battery is almost completely drained. At all other times, they sleep--meaning the system state is stored in RAM, and the machine goes into a low-power mode. Waking from sleep takes less time than restarting and restoring from hibernation.
But having the machine go fully off instead of just to sleep really can improve battery life. And because solid state drives start up exceptionally fast, restoring from hibernation is quick (five or so seconds, in my experience) compared to restoring from a standard drive.
There are several utilities available (such as Jinx's Smart Sleep, which installs as a preference pane and is Snow Leopard compatible) that will cause your Mac to hibernate instead of merely sleeping. Remember that when you want your Mac to emerge from hibernation, you must press the power button, rather than tapping any key on the keyboard.