It changes how the clouds look to the camera. But it also changes how the clouds look when rays reach them from other parts of the scene, such as path-traced lighting of surfaces. You are unlikely to notice much difference, unless your GI quality settings are very low. These settings might affect PT render times, depending on how many rays reach the clouds, but the default settings should be pretty good and you should only be concerned about this if you've increased these settings much above the defaults. Having said that, I haven't tested all this as extensively as I'd like, so it might be worth trying things out and seeing what effect they have on PT render times.