They don't have to match. Right now I'm making a D9P where the middle square is all the same but all the rest of them are different. The middle square gets cut into 4 smaller ones and they are scattered throughout. They could have easily been different and the effect of the quilt would have been the same. I'm using 3.5 squares (3"finished) which is what I already had some from other projects. If you use larger squares then the effect will be different too.
There are many layouts for a D9P and also different ways to cut the 9 patch which also results in different looks. Before you start cutting look at some of the different ways to cut them.