It has been my experience that using Spell Check never replaces proofreading a document. I have proofed documents where the person has relied only on Spell Check and the outcome has been comical. A few examples of this are: A lawyer writing to a potential client writes that he has expensive experience, instead of extensive experience. I remember reading a document and instead of its they had written tits. Bloopers to be sure, funny yes, but professional - NO! Be sure you always proofread your documents and then Spell Check it as an extra step to your proofreading.
A word of caution when using Spell Check: Be careful when you are going through the document that you don't accidentally press Replace rather than Add to Dictionary or vice versa.