I agree with Edd and would add that you only tip a taxi driver if he goes above and beyond — helps you with your luggage, etc. The ‘servicio’ of 10% that is added to your bill is enough but, again, if the service is extraordinary (not common here, by the way) you might want to add a bit more. I tip $1 per bag to bell staff in hotels — or $5 if we have a lot of stuff between the two of us. And it is common to leave $1 or $2 on the pillow in the morning for the cleaning staff.