I'm a stickler for correctness, and really have to hold myself back from alerting people about mechanical mistakes that appear in their posts and gaining an unwanted reputation as a grammar or spelling Nazi. Really, the only time people may feel thankful is if you catch it before it is published.
As a general rule, the most professional image for a business calls for correctly written content, but in terms of what actually is good for business there are always exceptions to the general rule.
In The Joys of Yinglish, (pp. 60-61) Leo Rosten includes a story about mistakes appearing in a sign. I've modified it somewhat, but kept the gist:
The sign of a store read: "EVRYTING FOR THE KICHEN" One of the men passing on the street walked in and told the owner, "There are three mistakes in the spelling on your sign." The owner calmly responded, "I know."
Surprised, the man asked, "If you know, why don't you correct it?"
The owner said, "Why should I? Each day, several people come in to tell me there are mistakes on the sign. Once they're in the store, at least half of them end up buying something."
So you see in this example, spelling errors prove good for business because they achieve a marketing objective: they get people in the store where they can actually make a purchase.