It all depends how meticulous you want to be about battery storage and maintenance. Allegedly a battery's life can be extended by being meticulous (saw a test study somewhere that I think reported about 15%, can't locate it right now), but whether it's worth the time and attention is up to you.
The recommendation (from DJI and battery manufacturers and the experience of folks) is that if you're going to store the battery for longer than 24-48 hours, it should be stored at about 50% charge level.
Yes, the DJI batteries are smart enough to discharge themselves for storage automatically. Through the DJI config (through DJI Go), you can set how long they wait until they do that. I think the default setting is 3 days. But the auto-discharge takes a couple of days, so with the default wait it could take 4-5 days until it gets to storage level. Pushing the battery power button seems to reset the wait cycle. Again depends how meticulous you want to be, but it might be better to take it down to 50% sooner than that.
The recommendation is also to deep-cycle the batteries every 20 charge cycles. Deep-cycle is taking it down to 8% (be careful not to excessively discharge, it could damage the battery). The deep-cycle refreshes the battery's ability to hold a full charge. The battery doesn't deep-cycle automatically, and there is no reminder in the app for it (that I know of), you have to keep track of it. The common way to do it with just the Phantom and accessories is by flying the battery down and/or running the motors on the ground.
There are dischargers out there that can help. One was linked above. I like this one:
Phantom Angel The better ones can do both storage and deep discharges, and the really good ones aren't fire hazards (the sketchy ones have a bare unprotected bulb hanging from wires that gets super hot).
No it's not necessary to use a discharger. If you're happy with the pace of the smart discharge, and with flying/spinning it down for the deep discharge, you're good. The discharger can do it when you want, can discharge faster, and can discharge without putting more time on the motors.