Where do you want 90% lean beef?
This is partly down to taste. When you make burgers much of the fat runs out by the time it's done, so you need to start with a high enough fat content that the burger isn't dry when it's done. I've made burgers with 10% fat when that's all I can find rather than 20% (my personal ideal), and they are very tasty as long as you don't overcook them. I find the extreme of 30% which I've seen recommended in places to be too greasy, but again that's my taste - I find too much fat covers the flavor of the meat.
Where you have to be very conscious of fat, whether minced or not, is in dishes where the fat has nowhere to go, like stews, braises and casseroles. Very fatty meat in these can end up with a greasy end result, which is not to most tastes. I use around 10% mince and lean cuts for those types of dishes.