Amy, your agent has a fiduciary duty to you and should not, without your prior knowledge and consent reveal the appraised value of your home. Having said that, there might be an occasion where you could actually stand to benefit. For example, the appraisal clearly exceeds the current fair market value. This happens more often than you might think as, depending upon the purpose of the appraisal the value conclusion can vary dramatically.
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!