Actually, it can depend on the topic as well. For example, publishers of computer books actually prefer direct queries and are suspicious of agented submissions. I received this advice from two authors of computer books. I sent queries for a computer program how-to book to all the major computer book publishers (many are owned by the huge publishing houses). I got more than 50 percent positive, personal replies, asking for more info (in the end, no one bought it, but I did have serious back-and-forth with three publishers).
My wife wrote a book specifically aimed at expatriates, and submitted it to publishers who do a lot of overseas business books, and she got around 90 percent personal replies, at least 4 publishers considered it seriously and put her through the wringers for changes. She finally published it in Hong Kong. The lesson is, if you target your query to the right publisher/editor, they are often receptive. Our method was: look in Jeff Herman's Guide To Book Publishers, Editors... Then (IMPORTANT!) phone each publisher directly and ask the receptionist whether the person in Herman's listing still works there, and if not, whom to write to.
