How much salt is needed in the streusel topping?

Even Homer nods.
Yes, make with a very small amount the first time...pinch, 1/8 or 1/4 tsp salt.
If you want more, try on a second baking.
The function of the salt is to bring out the other flavors, not become one itself.
Yes, sea salt or kosher are both good because they taste good and don't have added chemicals. But for this purpose and this small an amount, even table salt will do.