You'd be suprised sometimes, especially in the molar region how many times you end up where the appropriate adjustment isn't to the newly inserted crown, but to the opposing tooth. If you have a situation where do to either tipping/rotation of the opposing tooth that creates the classical "plunger cusp" or a situation where there's hyper-erruption of the opposing tooth secondary to either an existing uncontoured restoration that was in the tooth your crowning now or some similar condition that allowed the hyper-erruption to occur, then very often I'd be adjusting the opposing as long as when I'm done I end up with my occlussal plan restored and continuous with the adjacent teeth.
*Most* of the time though, I'll be adjusting the "variable" that I just inserted into the equation, the new crown. But there are plenty of exceptions to that rule, and that's where you need to think about what the problem is, why it happened, and what's the most appropriate way to fix it (notice that I didn't say the right way to fix it, since often there's multiple proper answers that have much to do with your level of training and big picture goal for the restoration of that patient)