I shut down my laptop when I am done with it as I see no reason to leave it on, then again I don't use it much. Laptops suffer from heat issues caused by tight cramped quarters so I always advise my clients to shut them down when done or for certain at least at night if that is all they use. Desktops the same way shutdown when I leave the house or at night before I go to bed. Nothing worse than a punch drunk computer whose memory hasn't reset in a week trying to solve an issue on memory from a week ago.
Many years ago I seldom had a hard drive last more than 2 years. Since I started my current policy I have drives running that are 8-10 years old and have never lost one.
This is from a printout I leave with all my clients:
Here are some points I would like to make on this topic.
1. Even in sleep mode power must be applied to the system in order for memory to remain active. Even though data is written from memory to the HDD before going into sleep mode, power must still be applied to memory when asleep. Actually low voltage power is continuously applied to the motherboard even during sleep; only peripheral devices are shut down or put into a low power state. This means the power supply must also remain alive.
2. Current (no matter how low a voltage) does wear down wires, but even more so microcircuits.
3. Heat is generated so long as current is being supplied and heat dries out protective coatings and sheathing. Both heat and current will weaken solder points over time.
4. Electricity creates electrostatic environment, especially when the environment is already very dry such as winter months. This attracts dust which as we all know restricts airflow causing over heating both inside the case and around heatsinks, builds up in fans and causes them to fail.
5. Surge protectors are virtually useless (IMHO), but better than nothing at all. They will NOT stop a lightning strike, nor will they prevent a black or brown out.
6. Storms are unpredictable as are auto accidents, high winds, etc. All of which can cause a power failure or rolling black or brown outs. All of which can cause an immediate loss of power to a system. Leaving a system on 24/7 increases the likelihood of a system being effected by them.
7. A sleeping system still uses more power than a powered down system.
8. Monitors in a low power state and printers are just as susceptible to all these conditions.
Conclusion: Unless there is some very specific need for a system to remain on such as remote monitoring, or data syncing, shut them off.