The tagging guidelines can be found on the main meta site, here and they are also explained in the Help Center, here. Briefly, they suggest the following:
Tag from the general to the specific. Include at least one broad tag and also include some specific ones.
As a general rule, you should try and add as many tags as you can. You are limited to 5 and, if there are 5 applicable tags, you should use them.
You should avoid tags that don't stand on their own. For example, tags like crash or programs that need to be qualified by another tag (what kind of programs? What about those programs? What kind of crash? What crashed?) should be avoided.
You should try and pick popular tags, those that are used most often.
While you should not attempt to summarize the question with the tags, the tags should give an idea of what the question is about. Tags are used to sort questions into categories, so if a question fits into more than one category, it should be tagged with more than one tag. In addition, many users have "Favorite Tags" and questions tagged with them are highlighted. This means that if you use one of those tags, you are more likely to attract the attention of the people who are most capable of answering you.
So, basically, use as many useful tags as are relevant.
