Depends on the mod, the battery, and the experience of the vaper. Some regulated mods can fire below .3, but they shut off if things go wrong. A mech doesn't.

The link posted above by naturalgut is a great starting point for battery safety. Then go to http://www.steam-engine.org/ , click the battery drain tab.

In that link posted by naturalgut, look at the efest batteries in the list. One is sold as a '35 amp' 18650 battery, but is actually a rewrapped LG HE2 20amp battery.

Back to steam engine. Select the LG 18650 HE2 (2500mah) battery preset. Drop the atomiser resistance to 0.3. See how it's drawing 14 amps (assuming a full, fresh 4.2 volt battery)? That's within the 20amp max continuous discharge, so you're pretty safe. If you push the resistance down to 0.2 though, now you're drawing 21 amps - over the limit for that battery, and into potentially risky territory. 0.15 ohms and you're drawing 28 amps - well over the limit, and probably very unsafe. And all this time, the wrapper says 35 amps, and steam-engine says your .15 ohm build's gonna be just fine.

.3 ohms is plenty low and only draws 14 amps, so even a bad or cloned battery will probably be ok. But there's no hard and fast rule.

Never use batteries with peeling or torn wraps either - for some batteries, the entire outer casing is the negative connection (which is protected by the wrap, so it can only be triggered at the bottom), so a mech with a metal sleeve can contact the battery and hard short. That will definitely be bad.

I'm really not the best person to explain this! Google 'vaping battery safety' or something and you should find some videos/articles etc. You're fine as long as you stay above .3, save the riskier stuff for when you're more experienced. :)