It depends on the type of writing and the audience. Also, English is an ever-evolving language, and so some rules of grammar need not always be followed to a tee as long as meaning can come across. This is dangerous territory though, because if some rules of grammar and syntax aren't observed then a sentence may not make sense to the reader. If you don't make sense, then you aren't communicating - if you aren't communicating then the letters on the page are meaningless symbols. Take for instance Faulkner's stream of consciousness in "The Sound and the Fury" - he doesn't follow every rule of grammar (by a long shot) but meaning still comes across.
As a rule, people who do not adhere to grammatical rules seem unprofessional unless they have already proven that they know the rules and choose to stray from them for some artistic reason.