Sign conventions are not arbitrary in trajectory planning, though. You'll get very different results if you solve for a robot trajectory that moves from A to B while minimizing motor energy consumption, than if you solve for maximum energy consumption. You'll notice the difference - it's a mistake you'll only make once!

If we have some internal decision-making process that pushes us to make decisions that minimize a pain metric, the sign we attach to the "pain" number will make a very big difference, both in internal state and resultant observed behaviour.