With videocards, I'd say it is best to apply sinks on all memory chips. Without any kind of evidence, I would say that a videocard that is missing a sink will start to artifact before the same card that has all chips sinked, simply because one overheating chip will produce visual errors, regardless if all other chips are running fine.
But for RAM, I don't think the symmetry is very important, since system RAM doesn't really need sinks anyway. But if the RAM is actually overheating and needs to be cooled, I'd think the same unsupported principle I outlined above will apply.
