Why should you care about grounding your MacBook or MacBook Pro? MacBooks have an aluminum body, which is highly conductive. On one client’s laptop the first reading we measured was around 48 volts of induced current on the body of the machine — voltage that can transfer to you while you type.
The simple fix is to always use the grounded (three-prong) power adapter and extension cord, which channels that voltage safely to ground. When the battery is fully charged, switch it off at the power strip while keeping the grounding cord connected — that way the ground path stays in place.