I work in a business where we receive written material from professionals all day long.
If we receive something that contains obvious grammatical or vocabulary errors, it starts the thought process: "What else has this guy done wrong?" At the very least, the proposal is going to be gone through with a fine tooth comb to uncover the other potential errors. They don't usually come singly. And when competition is strong, that can tip the balance against the writer.
In addition, a writer who presents something for publication with such an obvious error (and it's not a misspelling, by the way, it is apparent ignorance of a commonly used English phrase) is demonstrating that he doesn't care enough about his work to proofread or simply step across the room to consult a dictionary. Not an impression you want to create when you're trying to persuade people.
The internet is a useful tool, but it is full of semi-literate sludge, and the process of winnowing out the sludge from the useful material is both quick and severe. Why put yourself at a disadvantage with something that is easily corrected?
Just a word to the wise.
FMCDH(BITS)
With such an artless phrase as “What else has this guy done wrong?” I’d wonder why you think you have any wisdom to impart?
Then again, you said you get work FROM professionals. You didn’t say YOU are the professional. So, since you obviously know your place, I guess I can’t fault you too much.
Now, if you are done worrying about petty minutia, did you have anything sentient to say about the actual topic... aw, never mind. I’d only wonder what else you’d “done wrong” if you’d bother.