I wouldn't know about show purposes but I do know that too much fluctuation in milking times will cause a difference in milk production.
Heres my schedule, and I do adjust it occasionally.
The days that I work are 4 days of 5AM to 2PM and 1 day of 7 AM to 2 PM.
I milk both Bootsie and Binky at 4:30 am and Binky again between 3:30 and 4:30 pm, Bootsies boys are with her after the AM milking.
The day before I work the 7am shift or I am off, I milk at the regular time and strip her out at 6:30 or 9pm so that I have a 12 hour "fill time" on Binky for the next morning. She has adjusted very well and her production hasn't dropped, if anything it's increased by a half cup. She is still giving me just shy of 2 quarts a day.
We milk about 4:30 in the morning and 4:30 in the afternoon-twice a day. And most of the time we stay on schedule-except for the weekend when sometimes we sleep in a little bit. But even then we dont milk to much later than normal. For show purposes you'd want to stay on schedule so your does have milk in the show ring. Every doe handles the stress of the show differently and sometimes will not have as much milk as they would if they were at home where they are most comfortable.
We don't have a 12 hour schedule...life doesn't allow us to. Most generally I am out to do chores in the morning between 8:30 - 9:30(I am trying to do better about getting out there between 8:30 - 9:00). On Sunday mornings we are out by 7:00 - 7:30. Chores at night vary the most....depends on what we are doing that night. Normal night chores are between 5:30 - 6:30, but on nights like tonight when I have something at 6 I am out by 4-4:30.
Our girls still milk good, though. Doesn't really affect them.
On days that i work during the week i milk at seven am and seven pm, though the pm sometimes chores get done a little later for the purpose of milking later the next morning. I work from nine am to six pm most days. ON saturdays i work from eight thirty to five thirty, so i milk at six am and six pm. On sundays i work from ten to four so i milk at eight am and eight pm. No metter the day i try to keep milkings twelve to fourteen hours between.
i milk at 6:30 am & 5:30 pm. i can let them go over an hour on way or the other with out any change in productions. when i do shows i will milk that morning & then i judge how they look as to when to milk at a show. usually i will let them go 14hrs beforwe a show. but all are different so you have to check each on & go by how fast or slow they are making milk. there is no set rules(except at a show that has a milk out) it is more how each doe handles there stress of showing.
We milk between 8-9 am and then again between 7-8pm. I've never been very good at timing exactly 12 hrs. I just get as close as I can. If I have somebody deliver a single or someone who needs to up her production I'll milk 3 times a day.
We've been milking at 6:00 a.m. and 7:30. According to Storey's Guide for Raising Dairy Goats milking every 12 hours is not nearly as importent as milking at a regular time. But then, if you always milk at a different time, the goats will get use to that sceduale as well.