Lipos are not susposed to get below 3v/ cell meaning if you have a 3s lipo, then the pack isn't susposed to be bellow 9v. You will still get plenty of runtime cause the voltage in a lipo pack doesnt really go down until near the very end of its charge; that is one of the biggest benefits of lipo cells.
If you are running 6s then you would do 6cells*3V=18 volt cuttoff. You will still get more runtime than you would out of a Nihm pack because a lipo pack has more current so the system doesn't have to work as hard.
Think of it like this:
Top Gear sent a Toyota Prius around a race track as fast as it could with a BMW M3 but the BMW only had to keep up. The Prius got 18mpg around the track while the BMW got 20 because the BMW had extra power on tap so it was running 100%. Lipos are the same way, since they have more current, or power, than Nimhs, they are more efficient and have longer runtimes.