A generally good standard of writing is desirable for Web sites; spelling and grammar errors (or typos) are distracting and should be kept to a minimum.
Be honest with yourself. Unless your English is very good, get somebody more language-minded to check your site over, preferably before it goes live. Even if you are near-perfect, it's best to get somebody else to check: most people are very bad at spotting their own mistakes.
Finally, I should note that anybody attending my class who writes "it's" when they should have used "its" will be summarily executed.
