Now, I'm only six hours in to the game, but FYI unless there's more to this From Ashes DLC that I haven't seen yet I would definitely say you can totally skip it.
*Edit* - If you don't have the Collector's Edition that is. If you have the code to download it then it's worth inputting it, it's just not worth the $10 to buy it separately.
*Edit 2* - The gun you get is pretty badass, though.