It's so funny. I was reading this thread yesterday and saying, "Make sure you don't do that!"
Usually, I'm really good about it. Even if the queries are sent to a generic "queries" or "submissions" at blah blah blah dot com, I still look at the agent's bios so I know which one to address.
Yesterday, I queried the Carolyn Jenks agency, thought I had put in a query to a particular agent, got a partial request a few hours later, then went back in and saw that my query started with, "Dear Agent".
Who'd have thought my first request came from a "Dear Agent"??