If I am paying the price of grass-fed (and I usually do), I do not buy "ground beef". I get a cut that I want (usually 2/3 chuck, 1/3 sirloin or 1/2 Tri-Tip, 1/2 Hanger) and I ask the butcher to grind it up.
Don't worry about the fat content, worry about the quality of the cut.
I usually buy extra lean or lean, depending what is available or on sale. 7% fat. Here is some good info:
My local provider does not add any fat to his beef, it just gets ground up as it is. I don't know the percentage, but they don't shrink much in the pan. Compared to regular supermarket beef which leaks out lots of water and fat in the pan.
It depends what I'm using it for. Most of the time, I want the 85% lean, because I want the CLA and other healthy fatty acids. If it's going into chili or some other soup and I don't plan to pre-brown it, I get the leaner variety, usually 90%. I do that simply because the fat that separates in chili and soups is off-putting to me.
When buying GF ground beef (I usually buy other cuts) and I have a choice of "lean" percentage, I divide the price per pound by the lean percent and then buy the "cheapest" one. I figure I'm buying the protein. This seems like a good value approach, unless I'm missing something?