Angle of Attack
A 30* angle on a more static material (polyester) will put the entire load on the uphill edge of the strap. Only a load perpendicular to the tree will distribute the load to the entire width of the static strap, which is what I chose for my tarp ridge line application.
For the hammock, a nylon strap (more dynamic) will stretch on the upper edge and allow a geometrically proportionate distribution of the load across the width of the strap at an angle.
I just returned from the park to test my new hammock and nylon straps. I encountered odd angles and various surfaces on the trees. A green sprout was on one in exactly the wrong place but instead of breaking it off I just strapped over it with my softer material. The 6' nylon straps conformed to the many variables and I felt very safe. The "bend but not break" principle is at work in a nylon strap.
I also feel better about distributing weight on the tree over a greater surface. Trees have feelings, too. ;-)