I agree with what the others have written. The scrim side is the more durable side. With bed quilts, it should face down. With wall quilts or tummy time quilts, many quilters prefer it to face up. The shiny side is the scrim.
Scrim also slightly reduces how much the quilt will crinkle. The quilt in my avatar has scrim & was quilted SID. It had been washed multiple times & not blocked prior to the photo. You can get more crinkling by quilting closer together & using curvy lines. It was my first quilt & now I know that quilting that far apart is not great even if your batting can take it. There is also the stress on the fabric & thread to consider. I don't ever have quilting lines farther apart than 2" for quilts that will be used and washed on a regular basis.