Nothing beats the human brain. Even a skilled writer should, whenever possible, have another human being look over his or her copy before it's made available to customers, business partners, or the public.
This second pair of eyes does not have to belong to a professional proofreader or copy editor, although having someone like that handy is always a plus. It could be just about anyone.
It could be a colleague, of course. Someone who knows your business might catch factual errors or suggest improvements.
Or it could just be a friend or family member. Someone who doesn't know your business very well might point out a phrase or a term whose meaning is obvious to you, but might not be to the general reader.
Whoever it is, getting a different point of view will almost always result in some improvements to your writing: clearer phrasing, a better explanation of a technical point, catching spelling errors that you and your spell checker have missed, or any number of things. Two pairs of eyes are always better than one.
However, it is important to know when you do need professional help. If you're rolling out a new website or other mission-critical marketing materials, a marketing and/or writing professional may be needed. Poor grammar, typos, and spelling errors are no more acceptable on a website than they are on printed materials. An unclear sentence is just as bad as a broken link or a missing graphic.
So when you're getting your marketing materials ready for the public — whether it's printed collateral, a website, a press release, a resume, or anything else that has to represent you in the world — get a second pair of eyes to look it over. Use a thesaurus to mix things up a little. And when things really have to be perfect, refer (and defer) to the proper authorities: a dictionary for the correct spelling or meaning; a professional to whip things into perfect shape.