I always have a detailed plan, but only some of the fabric I will need going forwards. After having sewn a few blocks, I place them together to see how the color scheme is coming together. I never hesitate to change my mind, omit certain fabrics or colors, buy more, etc. to get the look I want. Every few blocks I repeat the process to see how the quilt is evolving. I always end up with extra blocks that won't be used in the final quilt, but I join them together and use them to practice quilting.