Do you exchange your money before you take the trip or after you arrive in your destination? Whenever possible I do this before the trip in case I need to pay for something in the other airport before I have the chance to do it.
Yes, it is always sensible to have a small amount of local currency when you arrive or go to the ATM and draw some out. Changing money for dollars or euros is easy to get from banks and post offices in the UK on the spot and at the airport as well. Sometimes on the plane, they can give you change in local currency too.
Before taking the trip. I do this because I don't want to go hunting for exchange houses in a country/city I know little about and because I tend to exhange when the rates are good - even if that means saving just a few measly bucks. And let's not forget that in some countries you can find shady exchange houses that cheat you out of your money - I know this because I'm living in a country where this is happening from time to time.
It's always advisable to changing your money [or at least some of it] before you leave. In addition to that it's always much better to get yourself a debit card you'll use while overseas. Mastercard is widely accepted in many places so if you pay for stuff with your debit card, you won't have to worry about changing any money once you arrive in your destination. But if you must [or when the need to get some local currency arises] go to a local bank. Their rates will be much better than you'll get elsewhere.
I usually change between $50-$100 before I go on a trip. It's convenient to have local currency when arriving at the airport, especially if there are some unforseen circumstances, such as delays or other small emergencies. I often get dehydrated on flights, and even just buying a bottle of water can sometimes become a challenge if you don't have the right currency or credit card.
