annual rainfall and swales

Do I need swales if I get 35 inches of rain?

Stepp get about as much as you and he has swales and pond so it might not hurt but then again you might not have the stepp 1000ft drop that he has.
You mainly need the water features if you cant keep water on your land/runoff. So in a urban valley lot you dont really have a problem with runoff
unless they did some earthworks to send the water to the street.

So seeing as how you are already in a forest setting I would say no. How steep is said area, do you have soil erosion problems.
could it be fixed by just aeration/infiltration daikon 3ft radish roots or by grassy groundcover.

If you ever experience drought you might benefit from swales or other rain harvesting earthworks.