My experience with samples closely mirrors Andys
As I do almost pure research and product development, and then turn the results over to production I almost never actualy place a significant order.
After our production group have morphed my work to fit space and other constraints they send the design out for mass production. (well ok I get a little more input than that) At that time the order is placed and the supplier benefits. I always treat the manufacturers as fairly as I can and never request samples without a specific requirement. I always follow up with info on design sucess or problems. One other thing- I make a point of filling in all surveys /marketing tools that they send. This courtesy applies only to companies that I use or plan to use. Terry