A few years ago, I wrote a blog post about why proofreading is important (http://blog.friendlycomputerhelp.net/why-is-proofreading-important/). Along with proofreading being important, it is also a good idea to have another set of eyes look over your work whenever possible. Even if you are like me and are usually noticing and pointing out mistakes, nobody is immune to occasionally making them.
Whether you are usually good or bad at spelling and grammar, it is a good idea to have at least one other person double check what you have written; especially if it is for school or business. We all make the occasional typo or accidentally use the wrong word. When you go over your own work, your mind will often see what you think you wrote instead of what was written. With another set of eyes, they can see what is on the document and let you know if you fell prey to human error.