Would you buy from a site advertising wigdets? I once saw a posted ad for "dahscund" puppies-- I don't remember the exact (mis)spelling, but do remember being impressed that they made three separate errors. Meaning: they know so little about the breed that they can't even spell its name right, either automatically and unthinkingly or by taking the time to look it up.
Spell checkers are a start but they can't do anything about usage (things like "your" vs. "you're", let alone "by" for "buy" and so on). There are loads of humorous blogs and articles illustrating the point.
It's best to have the page looked over by someone who has never seen it before. If you are small, that means leaving the Preview screen open and not yelling at your teenager/ spouse/ visiting neighbor when they look over your shoulder and say "Um, you misspelled 'Blowout Sale'". If you are big, there are people on your staff who are really good spellers; you just have to find them. I doubt it's their job title. Though maybe it could be ;)