I make my leashes with round braid, and when I get the length I want, I convert over to flat braid. I make the flat braid about 1 foot long and come back to my round braid and work the ends in to the round braid. I go for 4 "unders" to make sure it is secure. I put a cover knot over it (4 bight 11 part with a herringbone interweave), but that's more cosmetic than structural (I believe). If you go at least 4 'unders' and make it look good, you probably don't need to make a knot (unless you are already good at it), and I believe it won't be the weak spot in the setup!
Mike