My advice is, if you don't have programs running that need to be available 24/7, like telephone answering, internet downloading, television recording software etc, then you are better off powering down your computer for several reasons. 1, like you said, save power and money. 2, protect your computer and data from being damaged if there is a power outage or spike. 3, if you have a broadband internet connection turning off your computer when not in use limits your exposure to hackers and viruses. 4, save wear and tear on your computer components. Believe it or not computers do wear out, the most common failures are worn out capacitors in the power regulation circuits, turning off your computer saves wear and tear on your capacitors big time.
If you have a long boot up and shut down time you can use the Standby or Hibernate features which essentially take a snapshot of the system and then powers down the hardware, the system wakes up fast from this state. The only drawback with this is sometimes certain devices don't like to wake up properly, forcing you to reboot anyways if you need to use them. It depends on your hardware whether or not you may have any problems suspending your system. Older hardware may have limited or no support to suspend the system. If you do get used to using the features to suspend your system make sure to remember to actually reboot your system from time to time, windows pc's still typically need a reboot now and again to keep running happily....
Irie