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.