On Nasus, a “good” stack is only good if it does not cost your health bar, plates, or recall timing, because the report’s data says breaking your early gold matters far more than finishing lane with a prettier Q count.
Wither gets nastier after the opponent has already dashed or committed to the retreat, so many real Nasus kill windows come from casting W once they have chosen a path through the long top lane, not the instant they step up.
Once Nasus is online, stop measuring the game in stacks and start asking who can be kept inside your space, because his best midgame fights and side assignments come from making one carry or diver keep walking through Wither, Spirit Fire, and ulted Qs until they stop functioning.
Group when the fight will actually let you stay in contact with someone important. If the enemy comp has thin peel, your Ghost and R are up, or their diver/ADC can be trapped inside your space, your presence in a straight fight is worth more than another side wave. Stay side when they can comfortably kite you in open ground or stack enough crowd control that you never reach the part of the fight where Q matters. A good check is simple: if you can name the target your Wither is going on before the fight starts, grouping is usually live; if you cannot, side lane is often the cleaner job.
Nasus drains his foe's spiritual energy, giving him bonus Life Steal.
Nasus strikes his foe, dealing damage and increasing the power of his future Siphoning Strikes if he slays his target.
Nasus ages an enemy champion, decelerating their Move Speed and Attack Speed over time.
Nasus unleashes a spirit flame at a location, dealing damage and reducing the Armor of enemies who stand on it.
Nasus unleashes a mighty sandstorm that batters nearby enemies. While the storm rages, he gains increased Health, Attack Range, damages nearby enemies, has a reduced cooldown on Siphoning Strike, and gains bonus Armor and Magic Resistance.