There have been many 'safe' O2 sensor readings reported for DSMs. However, there is a growing consensus that O2 readings alone are not enough to guarantee safe operation of the engine.
DSM oxygen sensors should be thought of as more akin to oxygen 'thermostats'. They are designed to 'switch' states, from high to low, very rapidly, around the oxygen level that corresponds to a stoichiometric 14.7:1 air/fuel ratio. As long as they do this, there is no reason for them to be accurate anywhere else.
The principle of monitoring the A/F ratio is to check what the oxygen sensor is reading at A/F ratios that are greatly different from the switch point. However, the oxygen sensors may not be accurate at these levels, so any readings that are taken must be treated with caution.
Evidence to this effect is growing thanks to the introduction of the TMO datalogger, which gathers information about engine operation directly from the ECU. There have been many cases where owners have used the datalogger on their car, only to find (to their utter surprise) that despite sky-high A/F meter readings, they are losing power from not having enough fuel in the air/fuel mixture.
Another problem is that the O2 sensor reading shown by the A/F gauge may not exactly correspond to the O2 sensor reading inside the ECU. This is due to differences in the grounding points of the two devices, and can easily lead to a 0.1V difference, making the A/F meter reading 0.1V higher than the ECU reading. Thus, an owner might think that they are running a safer A/F ratio than, in fact, they are.
This does not mean that A/F meters are useless. Their fast reaction time and simplicity make them an excellent choice for monitoring relatively safe, early-stage modifications to DSMs. They simply have limitations that make them less-than-ideal for precision engine tuning, and new users need to be aware of them.
Having said all of that, authorities in the field have stated that 0.85V is the absolute minimum you can run. Most people prefer 0.90V or 0.95V, but running these levels is simply a guideline - it is not a guarantee that your engine is safe, or producing maximum power. This is because differences in engines, altitude, barometric pressure, gasoline, and other conditions all contribute towards varying this number.
The only sure method by which anyone can state that they must run a certain minimum O2 reading is if they have determined the perfect level for their individual car through experimentation. This type of experimentation is time-consuming. Many racers spend years perfecting their setups.