As a child I asked my brother how he can shower so quickly and got back "you only need to wash your armpits and balls". Turns out he was mostly correct, you can feel when grim builds up in the shower because water current changes. How often do you need to disinfect your shins? Water trickles down and most of us aren't wading around in swamps during the day. Systemic cleaning is good for teaching children and sterile conditions, clean what you need to.