I play my legacy with normal life span and aging on. I do occasionally cause a death for storytelling purposes.
I tend to choose a favorite offspring based on appearance or traits or just play the eldest. I will check in with the other offspring and try to make sure my favored offspring interacts with their extended family, but they are not my main focus. I use mods to make sure the played and unplayed sim lives stay interesting. :D
I roll various challenges into my generations to give myself rules and goals, but I'm also very lenient on those goals... use them more as inspiration. I'm currently on generation Earth Tones of the Extended Not So Berry Challenge with my 29th generation of my legacy. I will also let randomized traits or random occurrences in the game guide me on a story. For example, early in my legacy one of the children died from a dryer fire. His little brother grew up with an irrational fear of fire and never ate hot food.
My only hard and fast rule is to not take it too seriously. It's just a game.