If you email a company enquiring about a product or an order you've made with them and they answer your question, do you then email back to say thank you? Is it polite or annoying - as it's a waste of the helpdesk worker's time to have to read another (useless) email?
e.g. you ordered a product but afterwards realised the picture was a different version to the one that was listed. You query to check that they will send what was listed and not pictured. They answer to confirm. Do you then email back to thank them?