Well if you're talking whether it will hurt you in the search engine results for a term, I've never heard anything conclusive other than the word on the street which really can't be given a whole lot of stock as it's most often people guessing and the guesses become kinda viral
My guess here would be that the odd careless mistake won't hurt you but if you just suck at this stuff then yeah you probably need help
If you're optimizing for a term and you spell it wrong then you won't do well for that term although you will for the misspelled one as has been pointed out. Google frowns upon this but to my knowledge they don't have any penalties for it and in fact they reward you if there are a fair bit of searches for the stuff.
The reason why you want to pay attention to this is of course to give credibility to your site and there's nothing worse than spelling or grammar mistakes to make it look like you don't know what you're talking about. If you're challenged in this area so to speak and you can justify spending the money then paying for editing might be for you. In terms of spelling and grammar checkers, this will help eliminate mistakes and it's a good idea, but only if you know what the proper spelling and grammar is of course
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