There are no strict rules regarding the use of antiemetics. If someone rarely or never vomits, antiemetics may be unnecessary. But because vomiting can be a serious issue, the use of antiemetics is still recommended. (They are mandatory in assisted suicide with N in the Netherlands and Switzerland).
In the Netherlands, where there is the most experience with assisted suicide using N, metoclopramide is the first-choice antiemetic because it helps prevent nausea and speeds up stomach emptying.