I've done it both ways. I have collections of fabrics that'll go into one quilt when I find the right pattern for them. I don't worry about running out of fabric though. The amount of fabric in the collection determines how big the quilt will be. The fabric first approach tends to require math though.
It's also nice to have somebody else figure out how much fabric a pattern is going to take and just be able to go to my stash and pull fabrics.