Jim,
Although I purposefully use some slang and have "less-than-perfect" grammar skills, I still make a point to at least get my spelling right.
I always use a spell checker, but sometimes spell checkers don't always work (a good example would be too, to, two).
I will say that using the "right" slang or grammar has a lot to do with your target market.
For instance, If you are writing to a highly educated crowd (teachers

, lawyers, or doctors), you had better get it right if you want to build rapport or empathy.
If you are targeting folks with an 8th grade education, you better "not" use high-falutant(sp) "city folk" language.
Overall I do my best to use correct spelling.
I like Sugarman's idea though... might be worth a try in the future.
What could go wrong?
I guess you could offer $10.00 off the sales price and end up with fifty spelling errors instead of the assumed twenty.
In that case you'd have to give away your product for free or offer rebates on all purchases made...
What a disaster that would be!