I don't know about any rules of thumb, but I can tell you what I use.
For the kitchen towels, a nice finished edge might be reverse single crochet.
For the poncho edging, the choices are endless. A row of shells? A row of chain stitch loops followed by a row of nice treble crochet shells? Several rows of shells? Many rows of single crochet in a single color or in stripes? Maybe a filet-type edging?
Main thing to remember is when you come to corners (on the towels) or points (on the poncho) that you will need to make three crochet stitches in those areas.