A 2-cell LiPo battery has a fully charged voltage of 8.4 volts and the voltage does not quickly drop down under load but rather slowly .as it reaches discharge state Connecting 8.4 volts to a standard servo will result in burning up the servo in a short period of time. As already stated, you will need to use a voltage regulator to drop the voltage of a 2-cell LiPo pack to 6 volts.
Also for those interested, a 2-cell LiFe pack produces 7.2 volts fully charged but quickly drops under load to around 6.4 volts and remains there until it gets close to discharge then the voltage quickly drops down/falls off. Most servos and receivers and some ignitions can use a LiFe pack with no regulator.