That is completely correct. My farrier was out today and I love him! He describes step by step what he does and why! It's so awesome.
On one of my horses, my farrier doesn't even trim the insides on his hind feet. My gelding wears down the insides more then the outsides. So he trims half of the hoof and doesn't touch the other half. Just files it down. My gelding within 6 weeks, will wear it down again that way just because the way that he walks and stands. The outside of his hoof will flare out and that's the part that needs trimmed.