I would recomend that you do it before embarkation to increase your chances of getting your requested server as I was told that most table assignments are made several days before embarkation. It's important to note that the servers only have 4 or 5 tables, so requesting a popular server is more difficult than getting a cabana at CC
The Head Server along with the Server make the final decision - Sheldon told me that if he receives more requests than he has tables, he will select the passengers who he personally knows.
I should highlight that there are
many great servers on the Fantasy - although Sheldon is our favorite choice, there are three or four other servers that we have had in the past that we would be happy to have. When I submit my server request, I usually submit two or three names to insure I we get one of our favorites.
FYI: Sheldon will be on leave Sep/Oct 2023. I also showed this post to Sheldon and got a good laugh