Not until the letter has been read against the profession list and corrected if needed, the degree documents and translations are in hand, and any foreign credential evaluation is complete. A refusal is slower to repair than a week of preparation.
Fix the deciding document before spending a border trip on it
The port-of-entry route is fast, which tempts applicants to travel on the first version of the letter. The order should be: read the letter against Agriculturist and identify the sales problem; ask the employer to describe the job accurately; if the job qualifies, obtain a signed letter with title, duties, period and pay; gather the diploma, transcript and translations; and, if the degree is from outside the USMCA countries, wait for the credential evaluation. Only then choose the port of entry, ideally one with staff familiar with TN applications, and travel with the complete package.
If the employer prefers certainty, filing Form I-129 in advance produces an approval notice before the move, at the cost of processing time or a premium fee. Time the start date after whichever step is slowest, and remember that TN is granted for up to three years at a time and can be renewed while the same conditions hold.