also depends on how good your batteries are under load. the voltage drop of a battery under load is a function of the total internal resistance and current drain rate.

Newer high quality batteries (35-50C) can hold a much higher voltage under load as they have a lower internal resistance. As a result you can run a LVC of around 3.3v whilst still using approx. 80% of the batteries capacity and safe guarding the cells from damage. Internal resistance also increases as the battery nears the end of its usable capacity, exponentially increasing the rate of voltage drop under load and increasing the batteries temperature.

If you are running the battery close to its max continuous 'C' rating ..i.e: your battery is capable of 156A continuous..and you run until a LVC of 3.0V, the Lipo temperatures may increase close to or beyond safe levels (140F), causing damage to the cells (puffing).

Depends on your batteries, but to be safe I would say 3.2 or 3.3V. I have run 3.0V before with no issues .. BUT I was using batteries that were FAR more capable than the brushless system (i.e: drawing 17A from batteries capable of 66A continuous). Hence they only ran warm. Temperature is the killer.