I love scrap quilts! You could make it as random as you like. Some quilters just grab fabrics without looking and sew with beautiful results. If they pick a color that will sit next to the same color they just grab another color.
Or you could pick one fabric to add, say for your sashing or a center block and it calms the quilt down. If you use purple for example as a sashing color and a different purple in your blocks remember that if the purples touch they will blend in together. But if you put the purple in the block towards the center of the block and they don’t touch this doesn’t happen. Hope this makes sense.
Personally, I like the results of both methods. It’s all about what YOU like for YOUR quilt :P