I don't buy fat quarters either. In fact, I usually don't buy much fabric at the LQS unless I have a plan in mind. But I do have a lot of fabric..... a lot of it I find at estate sales, etc.
So, if you have the fabric selection you want to use in your quilt, just cut the pieces from that. If you need a few fat quarters, get them. It is really a matter of whether you have what you need for the quilt you are making and what you want to add to your stash.