Every trip out, I take a few shots of just sky any time that the sky is interesting, every kind of cloud patterns, high up, close to the horizon, morning, evening, summer, winter, anything. Now, I'm not actually going to admit that I might some day do something like salvage a poor sky image that's great otherwise by substituting a better sky, but... there are folks that sometimes do that, but not many of them will admit it. However there was a Very Famous Photographer some time ago that was dragged through a lot of caca because someone pointed out (very publicly) that his skies had some of the same clouds in different images. Uh oh!! Try to avoid that.