Blambot's Nate Piekos has written a comprehensive and informative article on the typographic conventions in comic-book lettering. A bunch of cool things regarding lettering I had never thought of:
Also called "cat's whiskers", "fireflies" or "crow's feet", breath marks are usually three little dashes stacked vertically that come before and after some sort of cough or sputter. The word with the breath marks around it may be italicized, lowercase or bold. There seems to be no hard and fast rule for these. I generally italicize and use lowercase and if the coughing gets really bad, I use bold. If you use an opening and closing set with no word in between, you get a symbol that looks like a tiny bursting bubble that indicates death or unconsciousness of a character. This is often used to end the text in a wavy balloon.
This is probably the biggest mistake seen amongst amateur letterers. An "I" with the crossbars on top and bottom is virtually only used for the personal pronoun, "I." The only other allowable use of the "crossbar I" is in abbreviations. Any other instance of the letter should just be the vertical stroke version. Although I would debate it, you occasionally see the "crossbar I" used in the first letter of the first word of a sentence, or the first letter of someone's name.
HT to Don for the find.