I would not do it for more than a few days and be very specific about what constitutes shadowing. In my view, it means that you follow the different providers in the practice on their busiest days and see how they interact with patients and staff members. Don't pass up a chance to eat lunch with the nurses as they will usually give you the real lowdown.
If the provider wants to use your services, make sure that there is a legitimate offer on the table.
I also recommend that you use your preceptorship hours as a way to do the same thing. That's what I did and figured out really fast which offices I did not want to work. I definitely don't recommend working in an office setting where a spouse is the office manager. I won't expound on that subject but understand that I will work for Wal-mart before that EVER happens again.
What say you? Any experiences?