I think it is basically a case of experiment and find what works best for you. That was kind of my point, being that you're a big ripped dude I was kind of surprised you asked the question lol. Personally I don't have the problem you are mentioning so I don't pay much attention if I start rounding on sit-ups. If that's the problem you're having it could be a weak back (weak spinal erectors/low back). I'm assuming you mean it rounds over when you start pulling the weight, not just getting into the starting position.
In any case, I would mimic the position you want to build first if that is your problem. Now that I think about it, I do usually keep the torso mostly straight when I do decline abs. I personally am a fan of ab rollouts because it requires you to fight the cave in both ways.