First, let me say my high school guidance counselor told me I wouldn’t graduate from high school unless I took something other than English classes. I am a sucker for good grammar, spelling, and the correct use of the English language. That said, I also think there is room for breaking the rules and adding personality to writing, even business writing. You have probably noticed some of my own rule breaking on this very blog.
HOWEVER, there are some rules that should never be broken. They won’t add personality to your writing, they will just make you look, well, dumb. Copyblogger put together an amazing infographic covering 15 of those unbroken rules. Read it and commit it to memory. Seriously.