That totally depends on the situation. If you invest in seismitoad's SpDef to avoid the 2HKO, how much do you need to take away from your defensive EVs? What does your team call on Seismitoad to check? If you are taking too much physical damage to the point that an another critical 3hko becomes a 2HKO, then I guess you need to think about whether the 3HKO from Typhlosion or 3HKO from [whatever else] is more important. If you have a secondary check to the other thing that you now get 2HKOed by, but your team otherwise gets wrecked by typhlosion, then you should still do it.
If you have something specific in mind, and your spread fulfills your specific needs, then use that. If you're lazy you rarely go wrong with 252/252 spreads, but if your spread makes him take attacks that your team needs him to take, then what else is there to think about?