|I can't say for sure how it is as an additive but I do know that washing soda is the principle ingredient in most powdered (and probably liquid) detergents, even the eco-friendly 'natural' ones. Now, that's not to say that if you put a lot in that it can't be harsh on fabrics, I don't know. But I figure that as long as you don't abuse of it, it can't be harsher than regular detergent...
And far as I know, baking soda is perfectly harmless. It's pretty darn gentle stuff, way more gentle than washing soda. But then again, I'm not sure it has the same level of cleaning power.