Rolling is the nature of stockinette stitch, but you could knit a few (3-5?) rows of garter stitch to start (K all rows), then add garter st around the edges (3 sts or so) to prevent rolling or do a seed stitch. I don't think washing the poncho will help with the rolling issue.
To check for other threads in reference to rolling (or any other problem you're encountering, go to the search menu and type in stockinette rolling...I'm sure you'll find help here if my idea doesn't work...these knitters quickly respond as a rule
The mod squad was here