This will generate the usual controversy here....
My own opinion is that there's probably no point in going out of your way to "buy Euros" before leaving. You can't win as an international currency speculator (which is what you're asking about). The dollar and euro will swing back and forth. Unless you really have a good crystal ball, odds are just as good that you would end up losing money if you buy in advance (and if you really are that good at speculation, you should be making so much money that you wouldn't care about saving a few bucks on your trip).
Unless you are either landing in Europe at some very obscure backcountry airport, or particularly unlucky, the airport you arrive at will have numerous functional ATMs all stocked up with plenty of Euros, and getting them there and then with your home ATM card will get you the best rate anywhere, period.
Bottom line: if you have leftover Euros from your last trip, bring them along so you have some pocket change when you land. As soon as you get off the plane, head for an ATM and grab a couple hundred Euro (at the best rate you're going to get anywhere on your trip), head into town and don't worry about it.
Really - ATMs are everywhere (including at the airports). They're easy to use. They're reliable. They give the best rates. Use the ATM when you land, and worry about other stuff.