Topping is usually done on the 3rd or 4th node for best results, usually I'll go for four if I can, especially if it's one plant. However I recently planted 3 beans in one pot so I only wanted 2 nodes on each, the first node was removed they were grown to the fourth node and I used 2 nodes on each. However it also depends on the plant.
If the first node is strong I might let it go and use four, if the first node is weak I might cut that and use 2 3 and 4th node. 3 is good because it's easier to control. Just using two nodes my Pink Grapefruit still produced 6.75 zips.
The Pink Grapefruit had leggy limbs and the other Pineapple Gum had short tight limbs, however both of them bent to my will and grew out just four main branches. After a few toppings you develop a feel for it, but Autos really really like to be topped, it improves their whole structure. So even two nodes works.