This is a question I am worried about as well. That's why I have decided against it. If you look at the servo spec sheets of JR stuff they never list a spec on 6V. I am not quite sure if the servos can actually handle it. I heard though that some of the servos can take it. Dunno.
I promise those 8611 and 8411's can take 6 volts. I know many people that are running both of those servos on 7.2 volt Lithium batteries that have been regulated down to 6-6.4 volts. The only problem some people encounter is that the 8411 will twitch slightly, but as soon as a load is placed on it, the twitch goes away.