It really depends on the boat and how it looks and how old it is.. you can see if its needed or not, can't you?? If you can't shouldn't be buying a boat on your own.
(not applicable to racers as they can look good and be 2 years old and be more worn then a Kazakstan stripper).
I hate the sail tests, I had a boat for sale in 2003, and was an attractive boat, too.
Once it went up for sail, I had a list of people wanting to buy it, just because I was offering a test sail.....I did go out with some, but soon I was refusing it...I hate what we call "smellers" they come in for the free ride...