I would say it is never right to smack a toddler .You have to remove them and reason with them as much as possible.
But I'll give you two examples of when I gave DS a single smack on the bum.( I hate to see a child being hit on the head)
DD and DS milling about in a shopping mall.I kept grabbing their arms/clothes to make them walk beside me. DS would have been 8-9 so old enough to do this.
He ran to get into a shop he wanted to go into -as children do. Unfortunately he ran right in front of a man with two crutches. The man didn't trip over him but it was a near miss.I gave DS a sharp smack to stop him short not to hurt him.Then I made him think about his actions and the results if he'd sent the man flying because of his stupidity.
He tends to kick stones ( the ones on driveways which migrate everywhere) when we walk to school. I keep telling him he might break a window or scratch a car. Or if something gets damaged he might get blamed.So when he did it again after warnings and he wasn't listening, I gave him one swipe. He stopped.
Some things though are SO dangerous. I was driving on the M6, just me and DCs. He was 7 she was 4.6. They were strapped in the back, had small toys, paper etc but were mucking about winding each other up.
I tried to ignore them as I can't drive when I'm angry and I warned them I would dump their toys. (We did stop for lots of breaks BTW)
Then one of them-don't know which one, they blamed each other- threw something (small toy) over my shoulder , at the windscreen.
I was %&@££" and stopped at the next service station , opened the car boot and put everyone of their toys in the bin*.
Whenever I ask the anti-smacking brigade what they would do in that situation ( on a motorway with missiles being thrown and the DCs not listening. "Do stop that darling or mummy might crash the car and we'll all be killed in a nasty accident" doesn't work.
Breathe