I think once you can reason with a child, any form of physical chastisement becomes totally unnecessary. As a stay at home dad to an infant and crawler, it was necessary to have some means of shocking the little one if they were about to put themselves in danger. A little tap or grab was effective as a way of stopping a fall off the sofa or similar, this is different to punishment as it is not directed towards causing pain. Pretty quickly, I realised that a sharp intake of breath had the same impact, made the baby stock still and focussed on me.
Physical intervention is necessary with kids, even just picking them up to drag them kicking and screaming when they have to go, could be interpreted the wrong way.
The should we, shouldn't we, smack them question is hard. Probably we shouldn't, and if we feel we need to smack, there are likely other things about our parenting that need improvement. In a healthy family where parents are working together, a one off transgression shouldn't be the end of the world. I was beaten as a kid, very rarely, it isn't such a terrible thing. Seems a little idealistic to imagine that all forms of physical violence can be banished from human interactions, it is a part of our makeup.
In a separated parent situation, this is a pragmatic rather then theoretical/moral question, anything bordering on physical correction should be avoided at all costs!