The advantages of using a regulator on 6V is that you will have consistent Voltage throughout the drain of the battery. After a charge, you may see voltage of over 7V..
If you regulate them down to say 5.7 they will stay that way from the first flight till whenever they get below 5.7V
Servo speed will always be the same throughout the drain and you wont have jittering servos after a full charge.

A regulator is so easy to install I am not sure why people dont use them.. Its as easy as plugging it in...