If you're flexible enough the steer tube is the limit but as you get older a taller head tube and spacers on the fork stem will help keep you from experiencing a large pain in the arse (see piriformis syndrome). Interestingly, although I don't see anything wrong riding with a rounded back if that's what's comfortable, something amazing I noticed, mid-foot pedaling as opposed to riding with the balls of the foot over pedal spindle: it seems natural to ride with a flatter back. As to why I'm not sure but it's almost like --e.g., do you feel you have to climb stairs with a rounded back? But, try climbing stairs with the ball of your foot on the edge of each stair -- it probably will feel more natural to lean into every step.