I had to double check my research now that I’m sending my YA ms out to editors and agents, since the books I have on my shelf only seemed to address this question for editors.
Maria Schneider, formerly of Writer’s Digest, seems to be everywhere on the web answering this question. She’s in favor, and hosted a post by Brad Thor (The Last Patriot), who is too.
What no agency will EVER leave off any of its expressions of preference, however, is mention of a policy forbidding simultaneous querying, the practice of sending out queries to more than one agent at a time. Some do have policies against simultaneous submissions, where more than one agent is reading requested materials at the same time, but believe me, the agencies that want an exclusive peek tend to be VERY up front about it.
So if you have checked to ascertain that the agent of your dreams — or at least the next on your list — does not have an exclusivity policy, you should assume that s/he doesn’t. End of story. Trust me, if an agent who does prefer an exclusive peek doesn’t want other agents seeing it, s/he will let you know.
I can see I’ll be going back to her blog often as she has scads of well-written, thought-out and in-depth advice.
And finally Writing for Children and Teens: A Crash Course (How to Write, Revise, and Publish a Kid’s or Teen Book with Children’s Book Publishers), by Cynthea Liu, gives a definitive yes. This book was published in 2008, in spite of the cover that seems to shout “1970s!”