It's very important to monitor for this (although subjectivity/calibration is an even more difficult issue). The reason is that in the most challenging conversations that pose the risk of lost business and damaged reputation or even financial loss the caller is particularly sensitive to sarcasm or insincere comments from the rep. They have their "radar" tuned up for anything less than a strong, positive promise and solution. They are often listening for "weakness" and ready to "jump on it".
Coaching reps around this skill is, in my experience, a bigger challenge than any technical skill set because it takes a lot of maturity and self-awareness for both the coach and the rep to affect change. Reps who master this are in a special category and are likely making a positive influence on your other reps already. Recognizing those people in front of a group also "teaches" others.