The only benefit of using "Sleep" or "Hibernate" is the reduced reboot time. Hard-drives still stop spinning, so they're still susceptible to (unlikely) damage (caused by dust) when they spin up as they come out of sleep mode. Hibernation mode uses the same amount of energy as when the machine is powered off, unless it's disconnected from the mains. Sleep uses slightly more energy. PCs don't use more energy to start from cold than they do in hibernation.

It used to be the case, say 10-15 years ago, that hardware was at greater risk when powering down/up, especially when you had to tell the OS to park the drive heads. That's no longer true, and modern PCs are designed to survive ~40,000 on/off cycles. That's 21 per day, everyday for 5 years.

So, should you turn off or go to sleep? It depends on nothing more than whether you want faster start-up times, or have the machine come to life when contacted from other machines on the network.