Yes. If I've understood your question correctly, then clearly the optimum scenario from a human rights situation is that you have what we would call an ex ante assessment. An ex ante assessment would take place before the agreement came into place, because then you're in a position to, as it were, deal with any negative human rights implications before they arise. From a human rights perspective, you do not want negative human rights implications, be they of agricultural workers or whoever may be affected by the agreement, and then have to react, have to mitigate, have to take action to compensate, or whatever.
What you want to do in an ideal scenario is to undertake the impact assessments before the agreement takes place--this is what the EU does with its social impact assessments--and then react accordingly, in terms of making amendments to the agreement, taking whatever other action may be needed. So certainly an ex ante assessment is the preferable option.