On Bonsai4me on the article on field growing he has argued that the tree must be left to grow until the base is the desired thickness and only then can you begin to cut back to introduce taper:
Elsewhere (Peter Adams' book) I've read that cutting back should be done continuously to maintain taper and limit the amount of large scars in future.Very often I have seen enthusiasts fail to understand that when developing a trunk for bonsai in the ground, the tree must first be grown freely until the base is the desired thickness intended for the bonsai that it will one day become. The example above perfectly illustrates that after trunk-chopping, the trunk base will barely thicken (if at all) until the new sections above it have reached the same diameter. By which point the taper (and the purpose of trunk chopping) has been lost.
So what do those with more experience than me here think is the best plan of attack?