Conventional wisdom for putting the tighter grain at the joint is because tighter grain tends to be stiffer. But that's not always the case and I have known builders who will put the wide grain in the middle if it is stiffer. But wide grain in the middle tends to look funny because we aren't used to seeing it that way.
There is an interesting comment I saw on a couple of YouTube video's by Dick Boak from Martin Guitars. How they book match tops (wide or narrow grain) in the centre depending on the body wood. One worked better for Mahogany, the other with Rosewood.
Thanks for the replies, everyone. Seems this is another one of those with no hard and fast rules. I went with joining the tight side, we'll see how it turns out.