Jack, as power lines are inductive reactance, power surges can be more devastating when power goes down them when it comes back. (V= L. di/dt).
I have a good surge arrest with a power switch. When the computer is not in use I turn it off and also the power switch.
Remember, ATX power supply is always on and memory are also with power on them. So even if the computer is "off" in fact it is partially on. That is why is important to have a physical switch to make sure everything is really off.
Also, if you never turn the computer off, only put it to sleep, memory is never cleaned, and performance drops down.
In other words, if the computer is not in use, turn it off. Better be safe than sorry.
If the computer takes a long time to boot, install a SSD and do a clean install (there is nothing best to gain performance than a clean install).