IMHO if watching a HD source then no sharpening should be neccesary so I set sharpness to 0.
If I use sharpness at all on SD sources it will only be very slight as too much will give hard edges and accentuate any artifacts.
KEF 3005SE, Onkyo 606, Panasonic BD30, Panasonic TX-P42V10B, Sky+HD