My car doubles as the "kid taxi" so I wouldn't want the other parents to see empties on the floor and begin to wonder. I bag them in the trunk, which also takes care of any ants.

Most found cans are a mixed lot, and I don't know anyone who drinks one Bud, then one Coors, then one Miller, etc., so I would hope a cop would notice that right away if he saw cans in a car.