I don't know whether I could find the old threads or not, but yes, this topic has been discussed before and some sharpen their blades, some don't; just replace them if they get bad enough. I've done both. Personally, I took mine off to sharpen them and I've done it two ways. I've sharpened them on a 6" bench grinder (a larger one would have been preferable in my opinion, but the 6" one was what I had), and I've put them in the vise and sharpened them with a 4.5" angle grinder (I liked that better than the bench grinder myself). I know some people just turn the brush hog up on its side and sharpen the blades with the angle grinder without removing them.
I guess you know that they come new not as sharp as a lawnmower blade. If you put a sharp edge on them, that's great for cutting grass, but if you are cutting heavier stuff, the sharp edge will get dinged up quicker and easier. If I remember right, my Bush Hog manual said to leave a 1/8" blunt edge, so how sharp you want them depends on what you intend to cut.
