This has been the subject of discussion before, and while there are woods which are considered toxic, one has to ask the question of its use. With a salt shaker, I would think it unlikely that any toxins from the wood would leach into the salt. I'm no chemist, and maybe with salt there might be some chemical reaction I'm not aware of. This is my disclaimer, in case anyone tries it and dies as a result!
However, the leaching aspect is usually more of a factor with moisture contact, such as with cutting boards. But even then, the question remains of how much leaching might occur, in what concentration, and is it really a concern?
But if you are concerned about it, maple is always a good route to go. It has been the industry standard for many, many moons for restaurant/bakery cutting surfaces. According to a health department report I read years ago when I was a baker, the tannins contained in maple actually hinder the growth of bacteria in the pores of the wood. And no known toxin to leach into food.
As for finishing a salt shaker, don't finish the inside at all. The outside could be done with darn near anything.