Posture is not only important while you are sitting down or standing up, but it also plays an important role in your training. What posture you adopt on a daily basis affects how well you train. If you are able to keep your shoulders back and down, naturally when you are training, you are able to keep your shoulders back and down.
Similarly, if you adopt bad posture, training can actually help you correct it. What will actually happen is that because of your training, whenever you adopt a bad position for your posture, your muscles and body naturally signal to you that you are in a bad posture and you make the correction automatically to get into a better posture.
So focus on keeping your posture day to day and you will reap the benefits in your training.