I started sitting up straight after having a series of back problems and learning through PT that a massive component of avoiding lower back problems is good posture. It can feel exhausting at first but it becomes a habit such that I stopped thinking about it.
On a slightly different topic, one of my pet peeves is women who don't stand up straight and whose shoulders hunch forward. I don't think they realize how awful it looks, from both physical appearance and confidence perspectives. I always want to walk up behind them and pull their shoulders back.
I do now, because you asked. Typically, if I'm sitting at home alone, I don't; but I do when sitting at my work desk. It's weird how I subconsciously relax and slouch with privacy. When I'm walking, I tend to have great posture, and people comment on it.