All that matters is that you try to maintain a "neutral spine". A neutral spine is a spine that conforms to its natural curvature. As long as you have a neutral spine, it doesn't matter what position, overall, you're in.

Of course, if you could strengthen your core muscles, that would help you maintain the neutral spine with more ease. Start with the abdominal muscles. Just practice contracting them while sitting at the desk.

So, if you need to lean forward to do paperwork, lean your whole torso forward, maintaining the neutral spine. This is also called "hinging".

I know this due to a lower back injury caused by lifting weights with improper technique, and I'm a fitness junkie. I also had a sedentary job sitting at a computer all day.

Neutral spine: https://mydoctor.kaiserpermanente.or...m75-488271.jpg

Hinging: http://www.mthoodchiropractic.com/wp...14-300x300.jpg