I follow back most people who follow me, because of the social aspect that 217mom mentions, and because on Twitter following people is much less of a commitment than it is on, say, a blog. On Twitter there isn't (or at least shouldn't be!) the expectation that your followers see and interact with every one of your tweets.
But like LadySlipper, I have gotten somewhat more selective. Accounts I tend not to follow:
feeds that seem to be nothing more than a "buy my product!" stream, with self-promo in every tweet
feeds for products (for example, if you happen to mention coffee in a tweet, you'll get a bunch of coffee businesses following you; I tend not to follow any commercial accounts other than chocolate and book sellers!)
accounts with no tweets
accounts that seem to be run by bots
accounts that are nothing more than quotations by famous people
hashtag overusers and abusers
people who tweet things I find offensive
And sometimes I don't add an account even though there is nothing offensive about it, just out of sheer fatigue.
My favorite Twitter feature is lists. You can create public or private ones--I have both. I have a couple of private lists that include the people whose tweets I most want to see (like my agent, my IRL friends, etc.). It's a great help in organizing my feed, and seeing more of the content from the people whose thoughts I most want to read.