This I admit is an unusual question. However, my question back to you would be. Have you invited them in? Either by requesting a reduction in taxes or by requesting a building permit. They cannot legally enter your home without probable cause unless they feel that you are in danger in some way. Which is the premiss under which some communities have gained access to neighborhood homes. In most cases, they will not enter your home without you being present or letting them in.
Hope this helps!
Good question. Not sure why the tax assessor would want to come into your home unless you've invited them by disputing your taxes or you have new construction. I've never heard of the assessor spontaneously wanting to come inside a home, but I don't know if they can without your approval. I look forward to hearing from someone who knows, but in the mean time have you called your township assessor to ask him/her?