I don't ever build below about .8 ohms, but lots of folks do. From what I've seen, most battery failures are a result of rapid discharge, either by the fire button inadvertently being pushed for too long, or batteries being shorted from metal objects completing a circuit, like carrying a battery around in a pocket or purse, along with keys, change, etc.
All of the batteries we use in mods have a maximum safe discharge rate, expressed in amps. If the maximum discharge rate is exceeded, the battery is in danger of failing. Batteries do different things when they do fail, depending on the type of battery. Some will explode or catch fire when they burst. Others, like "Safe Chemestry" batteries will burst, and discharge a pile of hot goo.
IMO, before anyone delves into the world of rebuilding, a person should have a basic understanding of ohms law, and how it relates to the batteries and coils you are using. You don't need a degree in electrical engineering. Just a basic understanding.
You also need to know what to do when something goes wrong, which likely will, at some point in your rebuilding career. Batteries don't usually fail without warning. The first sign is that either the battery or the mod you're using will begin to get very very warm. This means that something is wrong. At this point, stop using it immediately. If possible, remove the battery from the mod, get it out of your pants pocket, or purse, and put the battery someplace where it isn't likely to damage anything surrounding it. Because once a battery bursts, there's nothing you will be able to do to stop it. It's a runaway.
I may get flamed for this; but I'll say it anyway. There are some B&M shops that are offering coil building as a service; either for a fee, or as a free service to customers. I recently read about one shop where a shop employee set up a new customer, who had never used an RBA before, with a .3 ohm coil. That's crazy! JMO, but I don't think shops should be offering this service. Because if the user doesn't at least have a basic understanding of RBAs, what's safe and what isn't, and what to do when something gore wrong, they shouldn't be using RBAs until they do.