On this sort of grass slashing machine, it is the tip speed that does most of the cutting - the radius to the blade tip is quite large compared e.g. to a lawn mower, and at the operating rpm the tips are really whipping around. Grass, shrubs, small trees, etc don't stand a chance! If you get stuck into the blades with an angle grinder, you run a very real risk of overheating them and altering the design temper/hardness of the blades. Better to leave them alone, except to make sure that both blades are the same weight to ensure that the machine is balanced - one blade noticably different in weight from the other leads excess vibration and therefore extra load on parts like bearings and more wear and tear overall.
You will find that sharpening helps, but it isn't as critical as on a lawn mower. I have only sharpened my 6' slasher blades a couple of times in more than 20 years, and it has always cut effectively. Neater finish on "lawn" kinds of grass when sharp, would be the main observation I have made.