I usually look over my stash and if a quilt "comes to me," fine, I make it. If I don't have a quilt "come to me," I pick a fabric I like and head to a fabric store to find what goes with it better than what I have. Sometimes I even just start with new fabric. Each method works. I find that fabric I bought when I first started quilting, 4 years ago, isn't always fabric I am interested in now.
Basically I remind myself that fabric doesn't spoil, and fabric isn't getting any cheaper, so if I want to buy more, fine. I used to select my fabric before I selected a pattern. Any more, I start with a pattern. Not sure which way works better.
I'd say just do what you feel like doing. That is what makes it fun anyway.
Dina