What's the best (or required) way to reference letters when writing a book? For example: "She got straight A's all through high school." "I have to cross my T's and dot my I's." Not sure if the letters should be italicized, if they should be in quotes, or if they should have an apostrophe to show they're plural (T's and I's). Is there a correct way to go about this, or is it all just dependent on the author's writing style (as long as it's consistent)?