Lee Desser recently wrote a blog post: Are Happy Faces in Professional Communication So Bad? Lee brings up a good point… know your audience.
Places a would be no bueno
Cover Letter: You want to be professional. It is much better to be a little too formal with someone than too informal. I find it better if someone says “you can call me Al” (yes, I have heard that song) vs. “I would prefer it if you would call me Mr. Pollard.” It is always easier to become more casual.
Resume: You are stating facts, not getting cute. Be sure you are putting your best foot forward. You never know who is looking at your resume. Better safe than sorry.
Introduction Correspondence: If you are reaching out to someone for the first time, start off more formal. Don’t assume a connection between you two that is not there yet.
Places a may be OK or even help
Only someone with whom you have a relationship. But keep in mind, that relationship must be felt by the person on the receiving end. It can especially be helpful in continuing a more casual conversational tone.
Let’s say that during your on-site interviews a manager that you seemed to hit it off with jokes that he takes the closest parking spot because he is first in the office. When you write the thank you email to that manager you could say something like “I hope to have the chance to work with you. Don’t worry, I’ll still let you have the closest spot if I get there first :)”
But use a incorrectly and it can be no bueno for you. Personally I would avoid them unless you are 100% sure it will work. It is like sarcasm… when it works, it really works. But when it doesn’t, it pisses everyone off
Culture eats strategy for breakfast.