An interesting and long essay on the appropriate use of icons in your desinn. IA.Net sheds some light how many to use, what types to use, whether or not to use labels, and so on. And warns you to use proper context:
An icon stands for a section or a function, and represents what its label should communicate with a simple, clear term. Whether it communicates its function clearly depends on its conciseness as text in context—the notion an icon represents must be clear and concise in the context it is used. The most perfect icon will be of little use if the notion it represents is vague, or the context fuzzy.
Personally, I fall on the minimalist side, and always pin a note on icons and buttons to provide guidance to a confused user. Why? Because unless I use them all the time, I forget what they mean. Except for the most common ones, of course.
The main argument is that icons appeal to the emotions, while labels appeal to rationality. Adding icons leads to a happier user. Unless, of course, you overuse them, in which case your user is more frustrated.
And if you remove them all together, it leaves the user with a colder experience. So they claim.
Image plus Label
But don't think you can compromise by using both a label and an icon. Using both leads to spacing issues and a generally more crowded appearance. You've probably noticed this if you tried populating a button bar with both icons and labels. And the compromise of adding both subtracts from the emotional appeal of the icon.
Use the right number of icons
A retired Navy Chief Petty Office named Breezy taught me to cook in a restaurant a long time ago. He amazed me with his ability to free pour seasonings and still end up with a perfectly seasoned dish. I asked him how he knew how much seasoning to put into everything from small sauce pans to 5 gallon pots of soup. And his answer was a clear as the advice offered in this article: Just put in the right number grains to make it taste right, and no more.
And he was right. After cooking for several years, I learned how to free pour the right number of grains, a practice I still use to this day.
And I suspect that after enough practice, I'll know how many icons to use in any given situation.
Source: On Icons | iA