From my experience, the starting dates of post doc positions are flexible, though the degree of flexibility depends upon the funding source. Employers will be aware that you will be concurrently looking for other positions and that it is likely that you will need to start late or leave early. I think the key is to be open about the fact that you may be considering various positions at the same time. Try to avoid leading on one party, though, because it may have negative consequences later in your career. One case that anyone will understand is if you are offered a permanent faculty position. As such jobs are hard to come by, people will understand if you leave a post doc in order to take on such a position.