I have heard that drinking alcohol in front of cops during a traffic stop might be a good defense for a DWI. What if I keep a bottle in my car and then drink some after I pull out my keys?

Will that help prevent a DWI conviction in Fairfax County, Virginia?

We certainly do not recommend this as a way to avoid a DUI/DWI charge.  

The theory is that the breath test couldn't be used in Court because the test is just a measure of alcohol, and some of the alcohol was consumed after the driving was done.  

That action might prevent a breath test from being admitted in Court, but it does not mean you would be found not guilty of DUI/DWI.  If the police are able to show clear evidence of intoxication (based on your driving, field sobriety tests, and/or your behavior), you could still be convicted without the breath test evidence.

In addition, when the Judge considers your sentence if you're convicted, he or she might take into account your "clever" act of drinking in front of the cops.

Cops will usually say that a driver failed sobriety tests, but they can't really say that in Court.   They should be limited to specific obvervations about what they saw and let the judge draw the ultimate conclusion as to whether the client was intoxicated.    A skilled Fairfax County DUI Lawyer will always be arguing about all of the things that the client has done that are consistent with sobriety, but the absence of a breath test is not always enough to avoid conviction.