I'm guessing that at the level of Robbins, Ashton, and Taylor they were probably consulting with their peers and colleagues. I imagine Constant Lambert had a lot of input at the Royal. De Valois seems to have let Ashton alone, but she addressed what she considered to be deficiencies in other ways. She brought in Leonide Massine to do some things and work with the men because Ashton concentrated more on the female dancers and she thought the boys needed more attention.
When Robbins was with NYCB Balanchine probably came in for some look-sees, and on Broadway Robbins worked with George Abbott, who would have had a lot to say about what he thought did and didn't work.
(Peter Martins wrote some interesting things about apprenticing under Balanchine in "Far From Denmark." In one story he was having trouble with the ending of the ballet and kept noodling with it to little effect. Balanchine came in to watch. He made some changes not to the ending but to the section immediately preceding it, which set up the ending better and so made it more satisfactory.)