The correct starting point for sharpness is flat video luminance frequency response. The problem is, that point is not clearly marked on the scale. On some sets, flat is at the left while on other sets flat is in the middle. If flat is in the middle, then going to the left will make the picture more blurry than it has to be.
On average, increasing sharpness also tends to increase "ringing" echoes. If your set does not have this problem, then there is no disadvantage to increasing sharpness.
Of course you may adjust it any way you want.
Remember when lines and logos burned the TV screen? I was at a concert where a musical selection made extremely heavy use of about four of the keys of the piano.