I run four PCs at work 24/7 and two PCs at home 24/7. Most are IDE or SATA.
Also at each location is a www/email server which is a normal 3HGz PC with a SCSI 10,000 RPM HD and that runs 24/7.
Comparing the failure rates on the above, zero over 5-10 years, with failure rates on PCs that get turned off, there is not a shadow of doubt that by far the biggest reliability factor is leaving the thing running all the time.
One can't tell if things fail at power-down or power-up (think about it
) but when they do go, they sure as hell are found to be dead
at power-up. Since the earliest days of PCs (c. 1982) I have not had a PC pack up while it was running.
I replace drives when they start to make a noise. Trueimage, and clone the drive onto the new one.
The cost is in the electricity used, probably £200/year at home.