Please don't let the battery get below 2 volts....ever.
Good head room is around 3.4v (under load, particularly with sub-ohm setups...voltage is substantially lower).
Over drain on an Ion battery can cause it to reverse polarize...hopefully at worst it won't take a charge any more if this happens...but it's possible it could vent (very hot...dangerious).
Check the battery often...
If going sub-ohm...consider dividing its number of charge cycles in half, possibly overkill, but better safe than sorry.
Consider using a
current fuse.
For all day vaping without having to think much about it, consider a
Kick 2 button. That'll offer some 'vape it till it quits' battery protection, and will also allow you to 'fine tune' the wattage to your coils.
The Kick 2 is a really nice option for what I call 'pacifier PV use'...where sub-ohm raw mech use without the kick is more for 'I need a big and ultra fast nicotine dose'. A Kick would really come in handy if you don't always require massive clouds of eye watering vapor.