Don’t forget to thank those who help you in your research, either online or onsite. The main reason is that it’s polite and increases the chance that those people are willing to help others later. Who knows? You may even need their help again later and a thank you may make them more willing in the future to provide you with assistance.
And…there’s always the chance that your thank you note jogs the recipient’s mind about something else that could help you.
Librarians and clerks may be paid to assist people with materials and records. But if your treatment of them was unkind or rude it may be very easy for them to not provide you with that extra suggestion or idea that “dawns on them” after you leave.