Apply when the employer can truthfully describe lawful duties from the start date. If the licence will issue within weeks and the practice has non-clinical work, applying now is reasonable; if the practice has nothing she may lawfully do until licensure, the start date itself is premature.
Let the earliest lawful duty set the date
The question is not whether the officer will admit a veterinarian without a state licence; the credential rule allows it. The question is whether the position is real on the day she arrives. Hypothetical example: the practice group expects the licence in six weeks and needs her to review herd-health records, attend farm visits as an observer and complete onboarding meanwhile.
Those duties are genuine professional work the employer can describe, so an application timed to the start date holds together. If instead the group has no work for her until she can treat animals, an early arrival invites the question of what she will be paid to do, and a refusal at the port of entry becomes part of her record. Sequence the state application first, obtain the board's expected timeline in writing, and choose between the port of entry and an I-129 petition once the practice can state a start date it can defend.