in my experience dis pac fails in pairs of cylinders.(i've only changed one so correct me please)
The back of the dis shows 3 lumps. 1 "lump", iron coar i think its called, is responsible for firing 2 pots. If a lump fails two pots go down, 1 on each bank.
This meant that the engine was miss firing but balanced and sounded like a flat 4 cylinder engine, subaru style. As opposed to a v6.
However, when a lead goes down only one pot fails, (unless your incredibly unlucky.) This means the engine is not balanced, as you have 3pots firing on one side and 2 pots on the other. Result is the engine leaps from side to side violently as if its trying to jump out and walk. Quite a marked differance.
So, "in my experience" if the engine miss fires but is smooth with no vibration change dis pac. If its vibrating and jumping side to side, change lead(s).
Sounds like a rule, but as always with rules there is always an exception. Or my experience is not the norm...... See what others say.