It’s always a good idea to have your manuscript as completely edited and polished as possible before submitting to a publisher. And it’s also always a good idea to have someone else take a look at your manuscript as well—others spot things that you wouldn’t have noticed yourself.
Sometimes it can be a very good idea to work with a professional editor before submitting to a publisher. Publishers in general have less time and resources to devote to editing than they used to. I work for a small publisher, and we have to be really aware that we can’t afford to sign manuscripts if they need too much editorial work, even if they are promising and we love them. So working with an editor before you get to a submissions stage can really help you.
Not all editors are the same: a substantive edit is very different from a copyedit, for example. (A substantive edit includes “big picture” things like plot, character, structure, whereas a copyedit includes grammar, spelling, and punctuation.) The Editors’ Association of Canada has an overview of different types of editing. So if you do hire an editor, make sure both you and your editor know what your goal is with this edit.
It’s a great idea to get feedback on your manuscript and incorporate it before sending to publishers. This will take a different shape for every writer. One writer I know worked on developing and improving her manuscript through the Writers’ Guild of Alberta’s Mentorship Program, and afterwards hired an editor to help her get the manuscript to the next level. Only after that did she submit it to a publisher, where she was successful.
Another way to get feedback on your writing is through one of the many writer-in-residence programs. Many universities and libraries in the province offer the opportunity to meet with a writer for free and get feedback on a writing sample.