If, when in any foreign location, you opt not to use local currency, you will be ripped off everywhere you go. Imagine spending a few days in Dublin and paying for taxis, meals etc in Sterling. My advice to you would be to use your credit card for major purchases and withdraw a modest amount from an ATM to cover minor purchases. You will pay credit card charges but at least you will avoid Prague's ubiquitous and notorious 'commission free' bureaux de change.