The secret to unsweetened tea is to use controlled, short steepings. For the first 2 minutes or so, the flavor seeps out ahead of the bitterness. But every tea has a point beyond which the bitterness starts rapidly catching up.
Some cheap teas will have the bitter seep out sooner (allowing only 60-90 seconds for a flavor steep), but the vast majority will give you a good 2 min. Be careful with the timing & you won't need sugar!