I too have had this question come up recently.  There is no good answer and you are right about the County.  It truly depends on who answers.  The only thing you can do is to explain, like you do, that these are future bills and not yet available.  I have even tried to contact the "assessor" to attempt to get estimate figures.  No answers there either. 

  When the customer argues, I give them the County phone number and parcel number so they can call themselves.  That seems to the best option.  While it is not the best answer, it is the only one that seems to make the customers understand. 

We also make sure to make clear notation on the Title Commitment as to the nature of the assessment and the future billing of said assessment.  It reminds me of the Land Only taxes and flood of phone calls that inevitable follow when the parcel gets fully assessed.