I see cover letters and anything with an agent to be formal. Well, unless I were actually signed with an agent and had known them for years.... and then my email correspondence may end up using smiley faces. It's kind of like how when you're first hired at a job, you're very formal and professional, but once you get to know some of the people, you can loosen up (and potentially use smileys
).
Otherwise, I just remind myself that if it's not clear what I'm saying or what my emotions are without using a smiley, then I should really consider re-writing the sentence anyway. I don't want to leave any room for confusion of if I'm being silly, sarcastic, etc.