Be careful entering buildings, on buses etc. If someone in front of you stops suddenly, drops something etc, assume someone behind you is a pickpocket, we lost a camera at Prague Casle this way from a bag. It also happened on the Metro in Barcelona but my husband was aware this time and he might even have broken the guy's finger in his back pocket.
In answer to your question we found most restaurants, shops etc in Prague would take Euros, though we got a better rate using krone.