I buy fabric that jumps out at me... Some of these will probably just sit on my shelves for me to do nothing more than look and touch because it makes me happy. I have several stacks that are for future quilts, I add to them as I find them, this could take years/decades LOL. Other fabric is bought for specific quilts that I am currently working on and blenders I pick up whenever I find a good deal.
I think the hardest choice for me is how much of each to buy...I go with more is better for my longer range projects and the ones that really jump out at me. I know that when I do get around to using these, it may be impossible to find more, or I may have to pay high "ebay" prices once I do...