I have seen many people say not to go overboard buying dies. I don't necessarily agree. It depends on your style. I find that I use my cutter more now that I have more dies. I am not a long term planner when it comes to my quilting. If I want to make something, I'm not going to wait a week for the die to be delivered, I'll just rotary cut it. But I love that I can make a sampler quilt with the dies I have. I do have dies that I haven't used yet, I also have fabric I haven't used yet. To me, it is the same. Those dies will be there when I am ready for them.
I highly recommend keeping an organized list of which dies you have. My list is on my phone. If I am looking at a pattern or shopping for dies, I can know very quickly what I have.