I used to know the answer to tnat but it may have changed. Back in the old days it depended on whether you used Campy or Shimano components.
Campy recommended sizing the chain using the little/little method. While there's a 1 tooth difference in your little/little, you can't grow your chain by that little so you'd be good-to-go. The advantage of that method is that, if you ever switch back to an 11/28 cassette, you'd still be good-to-go.
Shimano recommended using the big/big method. Even so, I kind of doubt a 3 tooth difference would be enough to require shortening your chain.