Always if they help with the bag, or offer me a bottle of water. $1 per bag and $1 for the water. If they don't get out of the driver seat then nothing.

As far as cabs 10% of the bill if the cab is clean, no more than $1-2 if not. Also I'll add $1 per bag if they load it and remove it from the trunk. If they don't, then go back to the 10% rule...