I have no idea what is "right" per se, I just know what I do personally. I just take out their feed at night after they go to bed, anywhere between 5pm and 9 pm depending on the time of year, and butcher when I get up. I find that the crop is empty and intestines are about half the amount of what they would be if they were full. Makes it much easier to "harvest" the heart, liver, and gizzard. Plus, when the bird loses bowel control, it doesn't get all over you... yuck!
I do at most 6 or so birds... but if I was doing 250... no way would I even try myself.
I remember there being a link with what could be found in the intestines during certain times after feed withdrawal but not sure where it is. 12 hours is sufficient for my tastes. It saves me about 5 minutes each, and half the mess, compared to if I forgot and had no food withdrawal. It is also not as easy to nick the intestines when they are empty.