I leave the fabric swatch in the glass of water for several hours. Dye bleeds show up as wisps of color in the water. The idea is to weed out fabrics that either (1) did not have dye set properly by the manufacturer or (2) have a lot of excess dye that the manufacturer did not rinse out adequately. These are the fabrics that need to be prewashed or, if the dye bleeding cannot be stopped, discarded.
Most fabrics these days are colorfast. It seems to me to be unnecessary to wash all fabrics just to make sure a "bleeder" fabric doesn't make it into a quilt. It's easier to test for bleeder fabrics and treat just those fabrics differently.
Regarding Synthrapol, it does require hot water to be effective. All of the quilts I make are for use in the home, so I have no problem washing them in hot water in the largest front-loader I can find at the laundromat (for their first washing only). Knock on wood, I've never had a quilt suffer damage from this treatment.
I would probably change my routine if I were making a show quilt that was not going to get heavy use in a home. In that case, I might prewash all the fabrics and skip the first washing so that the quilt looks pristine in the show. Considering my age, I have serious doubts *that* is ever going to happen!
