In my non-professional (really, I'm just a learner too) opinion, it really depends on what you are shooting and what you hope to achieve. Higher ISO simply makes your camera more sensitive to incoming light. In the middle of a bright suuny day, this may kill a shot, or create some sort of desired effect. In low light, higher ISO may pick up the needed amount of light but the higher you go, the more likely you'll end up with unwanted "noise" in your picture. I'm sorry to state the obvious but it's dead true. I tend to use as low-an-ISO setting as I can for the conditions.
The one good thing with a digital camera is you can waste many shoots trying these things out without consequence, just to see the differring results. Go out and select one subject in the sun,, trying each ISO setting as far up as you want. Then try the same thing in less light at sunset and right down to maybe dark and once again, look at your results. This way you'll soon pickup on what's happening. I would generally leave mine on 100 - 200 on an ordinary sunny day, IF, I had to setup quickly to get a quick snap. I have just bought a D600 and before that I had (still have) a D300 and already I've found there is a noticable difference from one to the other camera so I just experiment 'til I find the needed settings for the conditions. Even a quick glimpse at the LCD screen to see the shot you've just taken can reveal a little info on how good or bad your settings are... The true test is when you get home and transfer them to the PC to examine. I always believe that aperture, shutter and ISO form that most important eternal triangle for good shots. From there you can vary other things for effect or fine tuning, but those three will determine your immediate outcome and when you adjust one, you will alter the other(s) if you have your camera in a priority mode, like mine in aperture priority...