I'm crazy for fabric, and love having my stash. I have many themes going, and always seem to have what I need no matter what project I am working on. I also trade fabrics with other quilters, and if my friend is looking for a specific fabric she doesn't have, she checks mine. We have lots of fun. When I purchase a pattern or get one free on line, I usually have the fabric I want to use, with maybe having to purchase a filler. I then put it together for a later time. I love looking at my fabrics stacked up on my shelves and it is almost like artwork!!!! It also helps me be more creative.
I also love books, but I don't just buy a book that I intend to read. I have various tastes and it depends on my mood what I read. Same with my fabric. I have a large library with books that might not all get read, but I love having them, and I know they will be read eventually by someone.
I think whatever fits your personality, and comfort level. When I first started sewing, I was apprehensive about how to choose fabrics. I had lots of help from my LQS's and my quilting friends. It also helps to take classes, and get to know the teachers. I have learned so much about fabric and color from just listening and watching. Taking pictures also help with color schemes. There is no right or wrong way whether you build up a stash, or be more conservative and only buy what you need.