I always make sure I have cash, about 200€, left over from my last trip when I return. It doesn't cost me anything except interest between the trips.
But today, Wells Fargo is selling euro at $232.26 for 200 euro. According to Oanda, the exchange rate right now is 200 euro for $220.16. You might get that rate a an ATM, but, depending on you bank, you'll probably pay $222.41 (1% plus no fee) to $232.00 (3% plus $5 fee) with most credit union or bank ATM cards. So you are probably looking at maybe $10, maybe nothing, to have the money in your pocket.
I can't agree with Rick on this. Airports in Europe have plenty of ATMs, but they don't always "work".
I haven't seen one, but I understand some airports are now going to only Travelex ATMs, which cost more than bringing the money with you.
I've heard stories of people arriving in Rome when the workers who fill the ATMs were on strike, and none of the ATMs had any cash.
Years ago, I arrived at FRA and went straight to the nearest bank ATM. I had three ATM cards with me - one that I had just acquired and tested the day before. When I put it in, the machine returned it and displayed, "call you bank". The second card - same thing. Now I'm getting a little concerned, but at least I have the 200 euro left over from my last trip to tide me through the weekend before I call my bank. Fortunately the last card worked. The reason for the failure of the first two cards was purely administrative and would have been rectified with a phone call, but still, I was glad I had my reserve.
If you are so sure that you will always find an ATM, before your next domestic trip, take all of you US money out of your wallet and leave it at home before flying.