Is it better to leave my computer's LCD screen on or have it turn on & off a lot?

It's better to have it turn on and off. LCD screens have issues with power-on hours but not cycling. To maximize the number of useful power-on hours, turn it off when you're not using it or set the power management settings to shut the monitor down when the system is inactive.

well, this is the same case with me, what i do is i turn it on and off frequently because my friend suggested me that i do not leave my PC on for the whole day..........................i don't think this helps but ohh well................................

Turn it off; LCD screens don't suffer nearly as badly as CRT monitors do, and it will save power. They also "warm up" much faster, so it's not as inconvenient to do.

choose the default of Windows in power mode... dont worry because backlight lifetime depends on your LCD quality like brands and... it doesnt have anything that how you use it !

As the others have said, go with the sleep option. Sleep mode is not as rough on a display device as a hard power off, regardless if you're talking about LCD or CRT monitors. I've never seen an LCD fail due to being powered on/off too often, at least I don't think I have. Just think of a regular light bulb. It is better to turn it off/on many times throughout the day so that it is only on for a total of 1-2 hours, than it is to leave it on the entire time. The bulb will surely fail from the hours of use before it will from being turned on/off frequently...

With the use of screen savers it can be left on. Power saver will turn it off for you after a time you set. Most commercial users leave them on with a screen saver running. There is a power surge when one is turned on.