OK, grammar... I'm a big fan of grammar, and it irks me that people abuse it so casually. I've even gone so far as to compose a list of my grammatical pet peeves on my Web site.
I had a good English teacher in high school. Mrs. Buice forced grammar lessons upon us whether we wanted them or not (and, of course, we didn't). We knew whenever a grammar lesson was imminent because our teacher would be wearing what she called her "grammar smock." We'd groan and resist, but look at me now I can write coherent sentences.
Mrs. Buice instilled a fear in us that English 101 was designed to weed out students who didn't belong in college. As Sugarmama pointed out in her blog today, the professor in a freshman English class doesn't have time to teach you something you should have learned long before. Instead of holding your hand through grammar lessons, we were told, college professors would simply fail you if you turned in a paper rife with errors.
Unfortunately, this is not the case. It's true that most professors won't bother to take the time to go back and re-teach grammar. Simply flunking students, however, would lead to lower retention rates. Instead, inept students are allowed to coast by, shuffled along until they're someone else's problem. Now, out in the professional world, those people who never learned high school-level grammar are inflicting their warped speech and writing on the rest of us.
Part of me wants to say, "big deal there are editors for that sort of thing." But I've been an editor (I still am, in some sense), and I would much rather fine-tune someone's style than fix elementary errors.
Not everyone has the luxury of an editor, though. And when you're the person who's making those grammatical errors, you probably don't know you're making them. As such, people end up sounding ignorant without even knowing it. Of course, they only sound ignorant to those of us who know the rules. I cringe at the thought of someone from my company sending out an e-mail that doesn't adhere to proper grammar. Someone else is going to read that and form a poor impression of our company.
Many will argue that grammar is a minor issue. So long as the message is conveyed, your goal has been reached. Yes, I understand what someone means when they say "we might could lose weight if we ate less candy bar's everyday," but I stumble on it nonetheless. I know that I'm not the norm. Most people probably won't take note of the slip, and of those who do, most won't care.
So, no, not everyone will think you sound stupid for making grammatical errors. But someone will. On the other hand, no one will accuse you of ignorance for getting it right.