It depends on the situation. For example, here's one of the Terms of Service from the model car forum (make note of the #1 result! ) I run:
7. Posts should include proper sentence structure out of consideration for our many members who do not speak English as their native language. That means using capitalization, reasonable punctuation and no excessive abbreviations. Posts should be structured as if you were writing a letter to someone instead of sending a text message. The goal is not to have perfectly written messages, rather to encourage a writing style accessible to all members, including those who don't speak English well, or who have to use translation services.