I think having a plan and buying for current projects to round out is best. I have a large stash which I am working from and I am focused on my current projects. I have one to finish by August. I have another to finish by November. I have one to finish by next March. It is an important one and will use a lot of different fabrics. I am also working on little projects to fill in and a Halloween quilt that I want done but not required by end of September.
Your tastes change so buying for current projects is best. I am currently using my stash to make blocks for fun that will go into charity quilts.