you're wasting more electricity by shutting it off.
The ibook uses less power than a digital clock when asleep, which is far less than the gulps of power required to shut down and reboot the computer once.
Plus saves time. Waking from sleep is a lot faster than booting.
You do realize that's a myth your stating, right? The amount of electricity used in turning on and off a computer is insignificant compared to the savings of the computer being off for long periods like overnight. Sleep mode is good if you plan on getting back to the computer shortly, but if you're done for the day, don't feel bad about turning it off.
Here's a variety of google results that talk about the turn off/turn on and power savings myths. 
