There has been some debate in recent years whether a J- or S-shaped spine is better. Supposedly the J shape is easier on our discs, though you need strong core muscles to maintain it. The military posture - straight spine, shoulders back - is close to the J shape. You’ll also see weightlifters exaggerate the J shape when doing squats to help avoid back injury. Though everything I’ve seen is anecdotal evidence. I haven’t seen any objective studies comparing these different postures.
Personally, I’m in the J/military posture camp. I have found that my back feels better when I pay attention to my posture and try to hold that shape. And I hate car seats that force me to have a more S-shaped curve.