Welcome to the board and the world of quilting! Your quilt, your rules! You can do anything you like. I am currently working on a quilt where the center is a flannel panel and in order for the quilt to get to the dimension I needed I added different size borders, only mine are wider on the sides rather than the top and bottom. Here is a picture, the black border closest to the panel is wider on the sides and so is the border outside the pieced pine trees. It works for me but with this picture you can determine if you want to do this or add a different fabric to just top and bottom in one of your rounds.