I get this question all the time at conferences. So let’s break this down….
It does happen sometimes that a querying author will let me know that their material is currently under review at a publishing house (usually because they had an invitation from attending a conference). This is acceptable, as you long as you let the agents you’re querying know. However, while it’s not the end of the world, I wouldn’t suggest on planning to submit to agencies and publishing houses simultaneously while you’re trying to find an agent. Here’s a few reasons why:
(1) It could be a deterrent to an interested agent if we learn it’s already been submitted to multiple houses (that’s fewer opportunities we have to get it sold). Conversely, if you have an editor request your work from a conference–but didn’t yet send it–that’s beneficial to let agents know, as we’ll pleased to know you have a bit of interest from a publisher that we can follow up on when/if we submit your work.
(2) Most agents are fairly hands-on editorially (I know I am!) and will work with you to make your project stronger before ever submitting to publishers. You only have one shot with a particular publishing house/imprint so you want your project in its best possible shape before submitting. Having an agent on your side BEFORE you submit will help make your project all the more appealing to publishers.
(3) When an author meets an editor at a conference and submits her work, she is typically only sending one project to one publishing house. Conversely, an agent is able to send the project to several houses at the same time in order to find the best fit for your work.
There you have it—three reasons why I suggest securing an agent before submitting your work to publishers!