It depends very strongly on how the game goes.
If the enemy team has a lot of AoE damage it is probably a better choice to split push and roam, since teamfights will only be disatvantageous for your team.
If the enemy is strong on kyting, you will Need to have a trap Play set up then teamfight. Kyting is in many (but not all cases) Focused on single target, save for any comp that can Build rylais effectively.
In any other scenario, engage in teamfights after you have enough stacks. YOu will always wanna try to stack more and more, stacks you have never enough.
Save your ultimates lategame either for the heat of a teamfight if you are in the middle of it all, or to save yourself if you are picked out then turn around and fight them like it's the ultimate battle between good and evil.
Nasus is a very situational participator for teams. His E makes good use for your ad carries since the armor shred is now percentage based BUT people can escape the zone easily, meaning you Need to lead a stationary fight to be effective or wither someone while they are CCed so the late-slow effect ticks the second they are free to move, shortening the Efficiency by time, but raising the possible versatility and combination potential for your W and E.
With Nasus, gameknowledge is essential. You are the raid boss in your team, means if you fall, your team falls, if you can take down at least 1 Person when you die, chances are Things will go smoothly.
Your goals are farm stacks and be the center of attention. Since you farm stacks hard you will become a threat noone can ignore, leading to the enemy losing if they do not focus you and you Q their carries to death, BUT also will mean they will lose if they cant split you apart and take down both you and your carry together.
Just beware of Things like grievious wounds. Nasus' sustain Comes from his passive which means lifesteal. Such can be cut out entirely.
When you splitpush, always be sure you are uninterrupted. Nasus is fucked when it Comes to escapes, meaning if someone can catch you while you are taking down a turret, chances are high you are dead and the turret won't even be dead.
With Nasus, your key aspect is one Thing, all this i wrote is taken down into one sentence:
- always know what you are doing