A little more on regulators...

A 6 volt battery when fully charged will actually put out over 6 volts. This can cause some 'twitching' with certain receiver/servo setups. A regulator will prevent that from happening.

You can also get regulators that are adjustable, say for 5.5 volts. You can use this with a 6 volt battery so you know you are getting the same strength out of your servos from the beginning to the end of the day. Servos put out a certain torque associated with the amount of power being delivered to them. Unregulated systems will have servos that are stronger and faster at the beginning of the day, and slower and weaker at the end of the day.

Using 2 cell lipos, you can set the voltage to 6 or 6.2 volts to get the maximum power and speed out of your servos, and be sure that you have the same strength and speed for every flight.