I personally have pretty good grammar, generally, although I don't nit-pick quite so much. I occasionally misuse hyphens and add too many commas, but I mostly stay on track. However, in manuscripts and query letters, you have to be super careful. Bad grammar in the manuscript might turn agents off, and you don't want your manuscript to be turned down because of grammar. That much, I know. If you're just not very good at spelling and grammar, and are sending out a manuscript, I'd suggest these simple tips.
-Make sure you're not mixing up homophones, or commonly-misspelled words
-Spell-check doesn't catch everything; I'd read it again to make sure that you're using the words that you want to use, not the ones autocorrect wants
-Have a friend or aquaintance who's good with grammar look over it to proofread it, because grammar can be kind of a big thing, like with agents
Even though the big things matter most in the long run, sometimes the little things are pretty important too. I hope this grammar post helps, at least a little bit, and -as usual- if you have any thoughts, let me know!