I recently read an online discussion of whether readers of paranormal romance are as put off by poor research or bad science as other readers. Whether you believe this is true or not, as a writer, you must consider this.
Imagine that a reader gives you a dollar’s worth of trust by reading your book. That trust means she expects you to give her certain things like a good story, interesting characters, and competent craft, among other things.
Every time your story fails in one of these elements, the reader takes away a bit of that money, and when there is no money left, the reader tosses the book without finishing it and will no longer trust you enough to buy the next book.
When the reader spots a grammatical mistake, she may take a nickel out of that dollar, or if she really hates grammatical errors, that error may cost you a quarter or the whole dollar.
Do you really want to risk losing that reader by being sloppy with grammar, science, or plot logic?