My last post on lead generation described the best techniques for identifying popular tags for any subject. You can use this approach to select accounts to follow, but that is just half of the solution. You also want these accounts to follow you back.
When you follow someone, the most likely thing that person will do is check out your account quickly to see if you are worth a follow back. If they see a tag in your description that shows you are passionate about that subject, they are probably going to follow you in return. It is a way of proving that you are a member of their community.
The other place that description tags are helpful is in a Twitter search. Most people think of search.twitter.com as a way of finding tweets, but when someone clicks the people link at the top of the search results, Twitter is going to rank your account high if it finds the right tag in your description. Think of this as Twitter SEO for gaining followers.