IMO you should check out the gamepress tier list, they have advise on which IV is supposedly the best. Not saying that I agree with it all the time.
The thing about nature is that you can have 21 different versions of a hero (grand heros notwithstanding). Not all stats are equally important, some boons may perform better than others.
As an example: Nino is IMO one of the best units in the game. Its a glass cannon. That means she can be easily destroyed by a melee unit. A +def boon will not change a thing. +atk, however, will give her more 6 atk points more than if she was -atk. And that can mean the difference between a OHKO and death. A +spd nature on the other hand will give her 39 spd over 33 spd (if she was -spd). With minus spd she wont double too many units and that makes her generally less powerful. Now imagine you give her fury3 and spd seal, that makes her 43 spd :)
Some units like Corderlia have brave weapons - they subtract 5spd points. Probably because they attack twice anyway. +spd cordelia will have 33spd. I you gave her Life and Death 3, she would be at 38, and with spd seal at 39, and as +10 unit at 43 spd. And that means she could hitcertain units 4 times.
Reinhard also hits twice but even the +spd version is so damn slow that you will never be able to hit 4 times. Well, maybe some units, I dont know.
When all is said and done, I wouldnt care too much about boons or banes, just be aware of what you got so that you can do proper merges if that need arises.