Once again the answer is, it is up to you! Pre searing is when you sear the chicken in a hot pan to give it a bit of color before it goes into the sous vide bag. In my opinion, pre searing adds another layer of flavor for the chicken to cook in. Post searing is when you remove the chicken from the cooking bag, pat it dry with a paper towel and sear it in a hot pan to give it color and crisp the outside.