I don't think it really matters much at all.
However, that being said, my invoice from my selling dealer, and my loan paperwork from my ag lender both have the model year of my tractor listed on them.
Maybe my situation is different, as there was no way for me to get a previous year tractor. My tractor model was a new series release for 2017. Purchased new in April of 2017. I'm sure there could be parts on my tractor that were actually manufactured in late 2016 to make the build date that the unit was assembled and shipped to the US, and then final assembled and shipped to my dealer.
Like I said, I really don't think it matters, but my documents have the model year on them.