I'm of the opinion that switching off & on does cause more wear and tear. I hear of more desktop PC failures than server failures. Most servers are only ever powered down when they need an upgrade and you'll find kit in many datacentres and computer rooms that are much older than 3 years! A more extreme example would be the IBM mainframes. These are rated as having 99.999% availability and the only time they are powered off if is you are going to dispose of them entirely or move them to a new site.
Fans tend to be a problem - they tend to accumulate dust quite quickly in the home and stop working or produce horrible noises.
Hard drives do fail - don't find out the hard way and backup your important stuff every so often!