Yes, if you can do it easily.
No if doing it is a strain.
But is it even possible to sit up without strain? Sure, once you see there's a third choice, you won't be stuck with the obvious "sit straight or slump" choice any more.
Let's first see why the obvious choice is not what it seems.
Slumping What is better for a person who, though normally active, sits slumped: to continue as he is or to force himself to sit straight? If he continues as he is, he will get progressively more and more slumped and eventually develop back pain.
Not good - but at least he's still able to do whatever he is able to do.
What about his obvious alternative? Sitting Up Straight His obvious alternative is to make the effort to pull himself up and sit straight.
If he does this, he is likely to look a little better but he will also inevitably be stiffer.
The apparently simple effort to sit up will make him stiffer, less mobile and even less alert.
Of course, if, like most people, you believe in sitting up straight, you won't believe what I just said.
If that's you, here's a challenge for you: after you've asked somebody to sit up, make a point of closely watching what happens to them.
Watch out for signs of stiffness; notice how their movement becomes more limited; notice how their eyes glaze over.
(Let me know if you don't see what I see and I'll come back and show you what you're missing.
) Becoming stiffer, less mobile and less alert is a poor reward for making the effort to sit up straight.
Worse still, if you needle a person often enough so that he keeps renewing his effort to sit up straight, he will get used to his limited mobility and get back pain even more often.
The third choice Is there really a third, better alternative? Sure there is.
The alternative is two-fold:-

1. Recognise what is making you slump in the first place.
   Once you tune in, you can begin to recognise that you have a lot of unnecessary muscular tension in your body.
   How do I know you have that tension? If you didn't, you wouldn't be slumping.
   
2. Learn how to stop doing that thing which makes you slump.
   Sure, you have to learn how.
   Sure, it takes work.
   Still, I'm betting you'll agree that the resulting freedom from back pain and bad posture is worth the initial effort.
   

It sure beats having to force yourself to sit straight every time you hit a chair - and it sure beats being a victim to mysterious back pains.