Just be honest, and don't be a dick about it. The whole "fucking cop pigs" is a stereotype, albeit a very common occurance, but no need for peaceful stoners like ourselves to down cops like that.

They can be exceedingly generous with letting you go, even if the only reason is to avoid paperwork, but they risk their jobs doing that.

And then there are the asshole cops.

But yeah, just be honest about it. Unless you have like 2 kilos in your trunk, then I wouldn't reccommend offering to let the cop search your car.