I actually prefer that they have just eaten when I butcher them. That's because when the crop is empty, it is really hard to find - it flattens out and practically disappears in there, and it's annoying to have to hunt for it. I never starve my chickens for any amount of time before processing, but yesterday I processed 4 cockerels that weren't mine, and one of them had a completely empty crop. Took me forever to find it! I use a kill cone and I've never had a poop fountain/explosion. Poop hasn't really been an issue for me unless I accidentally nick the intestines while cutting the cloaca out, but if I'm not too tired to pay attention, I can avoid that. Processing a well fed bird, when it's one from my flock, also offers me a nice glimpse into how it's doing - is it eating well, what is it eating, etc.