According to a ruling of the Supreme Court of Massachusetts issued April 5 in the case of Commonwealth v. Clint Daniel, driving stoned is still a crime and is not mitigated by the fact that you have smoked less than an ounce or have less than an ounce in your possession. In short, the less-than-an-ounce stipulation of the marijuana decriminalization measure passed by citizen initiative in 2008 only applies to marijuana possession as such, not to driving while intoxicated.
That could also mean that a police officer who sees you driving erratically while smelling funny has reason to believe a crime is being committed and therefore reason to search your car further.