After the discussion I thought about what everyone had to say on the subject and came to realize one very important thing - there are no hard and fast rules for querying. Sure there are 'experts' who will tell you how to do it and say if you don't follow their guidelines you'll never make it. You can add agents and publishers to your Google+ circles, subscribe to their blogs, follow them on Twitter, hide in their bushes and stalk them until the restraining order goes through, but it doesn't mean you'll end up with a better understanding of the industry as a whole.
I'm not suggesting we, as writers, should ignore everything agents tell us to do. Ignoring all protocol would end up with us alienating ourselves even further and missing out on any chance we might have. Instead, we need to be mindful of each agent's policies. Do they respond to every query? What is their normal response time? If you follow them on social media, have they mentioned they're behind? Do they expect the courtesy of being told if you've been give an offer of representation from another agent, regardless of what stage of the game you're at with them?
Figuring out all these things for each and every agent can become time consuming. If you don't have the time or patience to keep tabs on everyone, check out their policies before you query them. Query only the agents who you either really want to represent you, regardless of their response policies, or the ones whose policies are what you expect from an agent.
Now, get out there and query.