Eric,
You have to have more information than you have given to determine if your argument is defensible. As the house is already complete, gathering that information may be most difficult. Go back and read your 2006 again before the next job. You can't just take part of the code and make it fit. In the case of stating that something needs collar ties, you'll have to identify the slope, the rafter wood type and grade, identify how it is nailed to the top plate, whether or not the rafter are parallel to or in line with the joists, whether purlins are present, where the purlins suppoert the rafters.
Then you have to compare what you know to the tables. Then you can say that they are or are not needed.
You may be able to save face with your Client (forgive me if I am presuming anything here) by saying something along the lines of "I have misspoken. The AHJ has approved the rough framing and the final framing. I should have said, for additional protection against windlift and separation, you may benefit from installing collar ties in the upper one third of the rafter pairs."
This kind of thing happens to all of us. No big deal. It happened to me just two weeks ago concerning poorly designed and unsafe deck and dock construction that the builder wouldn't correct because there was no code to cover it.
