I usually put the mod on that increases iron damage onto the salvage tools because I find that they work great for quickly breaking down iron doors and breaking into locked stuff. Not only that, but most of the scrapping you do with them is iron, so it speeds things up a lot. But otherwise, I don't add anything else because there really isn't any reason to do so. Of course, once I have an auger, I just use that for breaking metal objects instead of the salvage tools. But I'll still leave that mod on the salvage tool just so it is faster for salvaging stuff.
And nothing on hammer or nailgun.