Safe? As low as possible. FX cpus have a Tdie of 62°C. That's not measurable by conventional means, you Need to use AMD Overdrive to use Thermal Margins. While that gives no Exact temp reading, it does give a clue as to how much thermal headroom is left with the current load.
Pushing voltage too high will trash that Thermal Margin as it's an algorithm made up of core voltages, load, socket temps, vrm outputs etc. The difference between 1.4v and 1.5v could easily be a Thermal Margin of Zero instead of 20, which means you are throttling the cpu and dangerously close to damage to the cores.