Yes and yes.
There used to be many reasons to turn off your PC, often at the end of a workday or before a weekend. These days? Not so much. For those who'd like a longer answer, here are the more common reasons for turning your PC off often and leaving it turned on full-time, and the techie replies for each:
"Leave your PC on to prevent the hard drive from wearing out."
When you start your computer, the hardware goes through a little more mechanical effort than usual. The theory was that this extra wear was bad for the equipment. But hardware is more durable now, and this isn't really an issue. By the time hardware wears out naturally, you'll be way overdue for an upgrade!
"Turn your PC off to prevent the hard drive from wearing out."
Yep, I've heard this same argument for leaving a PC off and on. Leaving a hard drive turned on full time is not a bad thing. Like the answer above, a good hard drive is durable enough to last longer than it takes for your computer to become outdated. Additionally, there are often built-in power settings that will temporarily slow down or stop a drive from spinning after a period of disuse.
"Turn off your PC to keep your monitor from wearing out."
This was also the original intent of the appropriately named "screen saver". When a monitor was left turned on, it would fall victim to "burn in". This is when an image displayed on the screen would "stick" - the image would still be there even though the monitor was turned off! You can still see this effect in any video game arcade: Look carefully at the monitor of any turned-off video game that uses a CRT, and you'll be able to see the faint image of the video game's main menu.
Today, computer monitors are very resistant to burn-in, although it still can happen with certain older CRTs. If you're really concerned, you can adjust your PC power settings to put your monitor in "standby mode". This will temporarily turn off the screen if you haven't been using it, and you can "wake it up" with a shake of the mouse or a press of a key.
"Turn off your PC to conserve energy."
This is still a justifiable reason to turn off your PC. A desktop PC, monitor, and associated devices will suck up 300 or 400 watts. That's a lot of light bulbs burning full-time. If you're concerned about the electric waste, you can turn off your PC. If you don't mind paying the extra dollars in electric bills, leave it on.