anna, what everyone said, with my usual observation that there is a lot of jazz in crochet of this sort. Whatever you do on the "vertical" sides (in your photo), in terms of number and types of stitches, just be consistent. Look at those end-row stitches, and if you poke edging stitches into say a DC in two particular places, poke them all that way. That way each end-row stitch will have a nice uniform relationship to the edge stitches you add.
I too like to start with a row of SC or HDC, usually in the same color as the body of a cloth, then add a contrasting border. That gives you a nice even invisible matrix to work the border into!