No such thing as duty free into the US. Lets see if I can make this even simpler. Stuff for sale in a duty free shop is free of taxes and duties AT THAT LOCATION. Nothing to do with importing into the US or any other country. When you enter the US you will be asked to VOLUNTARY declare all your purchases on a US form. It is your business if you decide to lie on that form. A custom agent will review and stamp that form as part of the entry into the US. Sometimes the agent will ask questions about the purchases and may ask to see an item. Coming back from Costa Rica this past February he asked to see the bottle of rum that we had declared. And sometime the agent looks at you, looks at your list and stamps it. And, of course, there is sometimes a random search. Never has happened to us, but the party in front was searched. Don't know whay. It all depends. If you are over it will not be thrown away, just pay the fine and the duty. And where you pack it -- checked or carryon -- makes no difference.
If saving a couple of dollars is that important to you, then do it. For me, it would be the inconvenience of being delayed is far important than saving a dollar.
PS -- I have never found duty free to be much of a bargain other than the convenience factor of buying liquids behind the security check point.