Your publisher would never publish your book without putting as many eyes on it as possible, as many fresh eyes as possible. Before publication every book should be seen by an editor who handles revisions, a copyeditor, and a proofreader (3 different people), as well as the author and whoever the author has look at it as well.
If you’re a querying author you should be sure that everything you put forth is looked at just as carefully as the published book. Because, in many ways, submitting your book a lot like getting published. You have one chance to earn a reader for life and not putting your very best foot forward is definitely going to ruin that for you.
I am not suggesting you need to hire an editor or a team of editors, what I am suggesting is that you find a fresh set of eyes. You already have a (or a group of) critique partners. They are your editors. Now you need another critique partner just for the copyediting and/or proofreading (I honestly think you could get away with just one of these steps at the submission process). I would also strongly suggest you do the same with your query letter. Sit down with your critique group when you start to write it, but have another critique partner (someone who has never read the book) analyze your letter as an agent would (the same way your publisher writes and reviews cover copy).
A fresh set of eyes can make all the difference when it comes to the strength of your material. I use them all the time here at BookEnds so why shouldn’t you?