To the nearest 5 centavos, I think it is. Doesn't help much when they don't have change for a 20 and the bill is 18.50. Few places will round it up that much.
I'm known at kioskos around my area and usually don't have a problem. They either let me make up the difference one time, or I let them the next time.
What's really aggravating is going to McDonalds and handing the cashier a 100 for a 45 peso meal and they tell you they don't have change...