There are quilters who prewash fabrics and quilters who rarely prewash fabrics. I'm in the latter category. It really depends on what you are making.
Fabric shrinkage is a factor to consider if you will be tying the quilt or plan on quilting lines quite wide apart (more than 6" apart). If, like me, you do a lot of moderate to close quilting (most of my lines are 2 to 3" apart), fabric shrinkage is not a concern. Once quilted moderately, batting takes control of shrinkage and does not allow fabric pieces to shrink more than the batting shrinks.
Bleeding is a more important consideration for me. Most fabrics these days are colorfast, but just one piece of fabric that bleeds profusely can ruin a quilt. If I suspect a fabric might bleed, I cut a small square of it and leave it in a glass of water for several hours. If I do not see dye bleeding into the water, I use the damp fabric to rub it against light-colored fabrics that will be in the quilt. If the fabric passes both tests, I don't prewash it (probably 99.9% of the fabrics I use). I do take the precaution of washing the finished quilt for the first time with Synthrapol and hot water in the largest front-loader our local laundromat has. This will take care of any minor bleeds.
In your case, though, instead of washing the fabrics you might just want to soak them in water, then iron. The soaking will reveal if you have any "bleeders" so you want to either soak fat quarters individually or grouped by color. They will not fray or get tangled up this way. You really do not need soap to determine if a fabric will bleed or not; it's the water that is more critical. If your only concern is shrinkage and you plan moderate quilting of the quilt, I wouldn't bother to presoak. If you ever do prewash a fabric, it's a good idea to starch it when you iron to restore some stability to the fabric (makes it easier to cut and piece accurately).