Either way, I'd test my machine on scrap first - it is a lot harder to pull out decorative stitches than regular. When you are doing that, it should be evident whether you need to use stabilizer, and whether the default width and length are giving you the look you want. Go slow and use your needle down setting to pivot on corners.
I've always done decorative stitching before assembly, but you may be able to get away with it using a blanket stitch.