Using gramatical errors in writing is a terribly common thing today.
But is there anywhere you can place bad grammar?
That's it: Dialog.
The dialog of books doesn't matter with grammar as much. A character could say "There's cows everywhere!" and that's okay. But if that were not dialog, it would be incorrect. For example, writing Loads of cows was on the farm is incorrect. But if Joe says "There's cows everywhere!", that is correct.
Always remember, though, that a set of gramatical rules still applies to dialog. You must still have your quotation marks, proper capitalization, and proper punctuation. Do not write: "theres cows everywhere! John said. Instead, use the proper marks and capitalization (even the apostrophe in the incorrect "there's"!): "There's cows everywhere!" John said.
