JOEL - There has never really been any "policy" on such trivial things. There were customs in the earlier days of the church that everyone seemed to strictly follow concerning what was worn. Nowadays there have been suggestions made by the church leaders that wearing white shirts and ties would help bring a sense of purity in the performance of priesthood ordinances.
In 1956 President David O. McKay said:
"I am not going to say much about the dress. We are not a people who look to formality, certainly we do not believe in phylacteries, in uniforms, on sacred occasions, but I do think that the Lord will be pleased with a bishopric if they will instruct the young men who are invited to administer the sacrament to dress properly. He will not be displeased if they come with a white shirt instead of a colored one, and we are not so poor that we cannot afford clean, white shirts for the boys who administer the sacrament. If they do not have them, at least they will come with clean hands, and especially with a pure heart. (Conference Report, October 1956)

Elder Jeffrey Holland said:
May I suggest that wherever possible a white shirt be worn by the deacons, teachers, and priests who handle the sacrament. For sacred ordinances in the Church we often use ceremonial clothing, and a white shirt could be seen as a gentle reminder of the white clothing you wore in the baptismal font and an anticipation of the white shirt you will soon wear into the temple and onto your missions.
That simple suggestion is not intended to be pharisaic or formalistic. We do not want deacons or priests in uniforms or unduly concerned about anything but the purity of their lives. But how our young people dress can teach a holy principle to us all, and it certainly can convey sanctity. As President David O. McKay taught, a white shirt contributes to the sacredness of the holy sacrament (see Conference Report, Oct. 1956, p. 89). (Jeffrey R. Holland, “This Do in Remembrance of Me,” Ensign, Nov. 1995, 67)

How something like this is enforced among priesthood brethren in individual wards is left up to the Bishop.