Want to sound like you went to an Ivy League school? Use proper grammar. Want to sound like an illiterate boob? Don't use proper grammar. It's pretty basic.
I must admit that I am not always the Queen of Grammar, but I know a link to someone that is:
Using proper grammar in resumes and letters to potential employers is a sure way to improve your chances of getting hired.
If you are unsure of your abilities in this area, ask for a friend to help you by proofreading your work.
Another fun site for the grammar minded is A Way With Words radio program.
Now... please don't start sending me emails correcting my grammar. I'm working on it.