Do I need a back-flow preventer for a new water to garden?
You likely need a backflow valve to meet code regulations these days. That said, I don't see any reason to have two of them. Whether it's at the hose bib, or at the well, it's doing the same thing for you. 

I suppose one argument for putting it closer to the well is that you block more of the water from back flowing, but if the pipe to the hose is rated for potable, that shouldn't matter.