There are 3 ways to challenge an assessment:
1) When a buyer purchases a home, they can challenge the assessment between January 1 and June 30.
2) Upon receipt of an assessment notice; the cycle is every 3 years in Maryland, and communities are divided into 3 grids so that the homes in each grid get reviewed once every three years, on a rotating basis. The next notices will come out July 1. You have 45 days to appeal. If your appeal is successful, you will not see the effect on your taxes until the next year. That's why you might want to appeal...
3) During the mid-cycle, I've written an article about it if you'd like to get the entire process... probably more info than you need right now, but someone reading might get something out of it. http://ezinearticles.com/?Appeal-Your-Maryland-Home-Tax-Asse
Most people are aware that the assessments are out of line in about 1/3 of the homes in the county. It shouldn't affect negotiations on a home.