Hi from Germany! I don't trust fabric that's not washed, especially batik fabrics, they tend to run. So I prewash ALL the fabric I drag home - just imagine you put an unwashed red fabric in a red and white quilt and the red runs! I would call that a catastrophe! And there are also fabrics that shrink! I taught beginning patchwork for 19 years and I made all my students prewash their fabric. I know a lot of quilters don't prewash - but I feel it is better to be safe than sorry!
Have fun! Greetings from - finally sunny - Tyrol!
Oh and should you have fabrics in your stash that are a mix of cotton and polyester - it is no myth that the batting, if you use poly batting, works through those fabrics!