Last, and close to cutover. Stakeholders first, reps last — always. Training the field weeks ahead means retraining them, because the configuration will still change; training them after go-live means the first week of adoption is the test environment.
Run leadership through it first so the decisions are settled, then a small group of reps as a pilot, then the whole team just before switch-over, followed by a week of white-glove support and office hours. Adoption problems are almost always sequencing problems.
Every claim above comes from work LeanScale published. These are the sources.