polyester webbing does stretch, and using webbing of lower break strength will result in more stretch. people have sucesfully used 800# webbing i believe without failure. i personally have tested polyester webbing from 1200-3500# and found the owf camo poly webbing (2000#) to be the best combo of low weight, and low stetch.
as far as how much strength you need, it depends on how heavy you are and the sag angle you use, this is influenced by weather or not you use a ridgeline and how tight you crank it. i would say you should be fine with any webbing rated to over 1000#, but like i said, weaker webbing will stretch more, if low stretch is important to you, use a stronger webbing.