Tension

When we stand and inhale are the abs supposed to be totally relaxed or is there supposed to be slight tension in the low abs

The goal is total relaxation on your inhale! There are different schools of thought, though, and if you find something else works for you, please share!

But don't the low abs tense to keep you upright or are you supposed to let go of that as well 

I don't think of that as tension, but it does take some engagement to stand with good posture!