Another favorite tool of the Internet Expert is the histogram. They like to show you the histogram of your photograph and point out features of it to explain to you why your photograph sucks.
Ignore these people.
In fact, ignore the histogram, for the most part. It can be a useful learning tool, but it's really not showing you anything that is not blindingly obvious in the image, so learn to see the image instead. By all means, compare with the histogram a few times, until you can pretty much visualize what it will look like for any image. This should take a very short time.
The only other time the histogram could be useful if for checking your images as you shoot. The histogram just tells you what your photograph looks like, after all, but sometimes it's pretty hard to tell what your photograph looks like when you're peering at the back of your digital camera. So, consider looking at the histogram instead when it's hard to see the image.
Anyone who tells you that the histogram has to start at the bottom and go all the way to the top is a twit. In the first place, they're gabbling away in technical terms about something that's extremely obvious visually -- a full range of tones, from blacks to whites. Presumably they're being technical to make themselves sound smart, or because having finally understood the histogram they think it is important. In the second place, it is simply not the case that every image needs black blacks and white whites. The tonal range has a profound effect on the mood of the image, and sometimes you want one of the other ones.
Of course you should think about the tonal range of your photograph, and what the emotional impact of it is. Adjust to taste. There is no "right answer" here.