I've been told to list my agents in order of preference and to start at the top with my dream agent and work my way down.
Then I've been told not to start with my dream agent because if there are problems with the query I don't want to test the waters on my dream agent, but to start further down the list and work up to my dream agent.
Then I've been told to query anyone except my dream agent and when I get an offer from someone to then query my dream agent stating there is another offer on the table (something I don't feel comfortable with).
There is no right way to do this. There's the way you do it (which you'll kick yourself for doing) and the way that Other Writer did it (which will make you think Oh, I wish I'd done that instead!) The reason is that the grass is always greener in the other WorryWart's pasture.
The real problem here is not who to query first, but that you've set your heart on a Dream Agent. Do Not Do This. No matter how fetching the shark may appear, she is not the right agent for every writer. Not even for every good writer. Not even for you.
Right now you want to look for a couple things:
1. Has the agent made sales, or if the agent is newish, are they apprenticed to someone you've heard of. That means a new agent who has set up shop to "help writers" and worked as a librarian most recently and hopes you'll help her set publishing straight on how to do things is NOT someone you want to query. That does mean a new agent who sits in the same office as Barbara Poelle, or Holly Root, or Joanna Volpe or any of the other amazing young agents in this industry is far more likely to be able to sell your work regardless of how many other projects s/he has sold. Trust me on this. I sit next to one of the brightest up and comers in a long time: Brooks Sherman.
An experienced established agent should have books you can buy in bookstores, or on a major retailing site. More than ten of them too.
2. Does the agent love your work? And I don't mean tepid "well, this is ok, but" I mean love with passion that makes Twilight look bleak. You want someone who loves your work and will be passionate about getting other people to read it.
3. Does the agent have a plan on where/how to sell the book. Here's where I'm very much NOT the dream agent for very good writers. I've read a couple darn good novels this year that I had no idea what to do with. I passed with great sorrow BUT the author deserves an agent who knows what the next step is, and I didn't. I mean short of sending it out to every editor I've drunk under the table with a cover letter that says "this is good, buy this" (Ok, I've done that but it's not really a strategy you want to encourage.)
The one thing you Do Not Want To Do is what you've already identified as questionable in that last line. If you get an offer, that's not the time to start sending your query (or worse, your ms) to all your "Dream Agents." We can read the chronology better than you can lie about it, and nothing ticks me off more quickly than "Hey I have an offer, you want in?" cause really, if you're so hot for me how come this is the first time you've sashayed by and dropped a hankie?
(well, ok, I'm not open to queries but you get the point)
Get your query in good shape, pick five names and start querying. It's better to get started than dither about how to proceed.