There's much to be said for this advice from Texas Star --- if grotesque grammatical and spelling errors appear in your query letters, they indicate carelessness and sloppiness in your work.

On the other hand, if clumsy typos and obviously unintentional errors appear in your manuscripts, copyeditors should notice and correct them. The problem is that most magazines have little or no competent copyediting or fact-checking, and some actually introduce errors --- some nitwit once changed ammunition to bullets making me look ignorant, in one of the last few pieces I wrote for that publication. (I can embarass myself easily enough, thank you, without help from some ignoramus...) The main thing is to be able to express an idea appealingly, interestingly, and authoritavely, in the always limited available space.