My advice is to make sure you have excellent storage facilities for a fabric stash before you start collecting. I have stacks of clear plastic boxes and, frankly speaking, if I were starting over again I would *not* buy so much fabric without specific projects in mind. My tastes in fabric have changed radically over time, so I have given away a lot of yardage just to make space for the types of fabrics I like to work with now. I still have so much fabric that it is difficult to pull everything that might work for a specific project; I dread having to move all those stacked boxes, rummage through them, and then replace everything in the closet. Even having given away a lot of yardage, I still have way too much of some fabrics.
With fabric getting more expensive every year, it is harder that it used to be for me to give away fabric. It is also getting harder for me to buy more fabric because I have less wealth (!) and the fabric I like now costs a lot more than the fabric I used to like.
I agree that if you just buy extra for every project you make, soon enough you will have a stash that reflects your tastes in fabric. Be patient!!! If you still have the collecting bug, I would advise purchasing only fabrics that you really *love*, not fabrics that you think might come in useful some day. I found that "some day" sometimes never comes!
Edit: I decided I should add one caveat, that my advice should be ignored if you are truly an artist. Ruth McDowell comes to mind. She has accummulated fabrics for many years and draws on them for every quilt she makes.