If you are talking about the assessed value for property taxation purposes, it is public information anyway and may or may not be an accurate reflection of fair market value of your house. I am working with sellers who submitted abatements to lower their assessments as well as sellers who have submitted abatements to RAISE their assessments (usually because of errors that unfairly reflect a low value where there is sufficient market data to point to a higher value). A lot of buyers will use the assessment value as a starting point regardless of its accuracy.
If your agent was handing out an appraisal that you paid for, that is a different situation. Not only does that information belong to you, but appraisals for any reason are likely to become quickly out of date.
A buyer's lender may or may not look at the assessed value. It is probably not a primary yardstick, but rather a verification. Certainly your agent should let the lender's appraiser know if there are errors or omissions in the assessment even if you have not yet notified the taxing authority. Your agent should have advised you what the market data might suggest to an appraiser with the hopes of ensuring that you did not price the property so as to increase the chances that it would fail to appraise for a buyer seeking a loan.
Chuck Braxton, REALTOR GRI
But that does not change the fact that your agent should still seek your consent.
It's also important to note that just because an appraisal for one buyer's purchase comes in low, that does not preclude that a subsequent appraisal wouldn't come in higher. The one exception that I am aware of is an FHA appraisal which runs with the property, meaning that that is the final word. Good Luck Amy!