In answer to your question, Yes, the assessed values have typically gone down in your area for the past couple of years. That is a delayed reaction to the values falling and homeowners object when they have to pay taxes based on negative values. In recent months, however, the market values have increased in your 22066 zip code area. When that happens, you can be assured that taxes will rise at some point.
The previous answers regarding market values vs assessments cannot be stressed enough. Many homeowners erroneously think that assessed value = market value. It does not. Market value depends on much more criteria.
For a market evaluation, please feel free to contact me. I would be happy to offer a complimentary market evaluation of your investment.