So the first thing you need to do is get your twill tape out. I just happened to have some pink to match my fabric. Put your front and back piece together at the shoulder with the right sides facing eachother (RST) and make sure you're stitching the twill tape close enough to the edge, like in this photo. Remember that shoulder seams need to be pressed towards the back so your layers should be as follows: twill tape, front piece, and back piece (always making sure your pieces are RST)
I cut off any extra length of twill tape, and finish off with the serger, you could also finish off your seam with a zigzag stitch
Press your seam open and your seam allowance towards the back.
Try it, and I think you'll agree that it doesn't take up time but it will make a difference on the end result, espcially with those extremely stretchy knits.
By the way, I can't wait to show you what I've made with this pink plaid fleece!