Assuming that your team does a great job building its estimates and using a probabilistic approach versus a deterministic approach, and finds that the scope of work will take between 12 to 18 months to complete with their most likely estimate around 14 months, is it better to target closer to the 18 months to build in some degree of assurance of meeting it or would it be better to target closer to 14 or even 13 months...and why? What are the pros and cons to either the seller or buyer?
This is not one of those questions that has a ready answer and probably violates the rules, but I think it is an interesting and relevant topic to PM worth some debate and argument, so I'm asking it anyways. :)
Addendum: Notwithstanding an initial risk evaluation, is there value in "forcing" a more optimistic target than what one might find comfortable? Can this be a productive and healthy challenge for the team? Can this be an effective way to control Parkinson's Law and Student Syndrome?