I used to, but not anymore. That's mainly due to personal preference and the fact that I've got alot of logo's.
I do, however, sometime put logo's on letters (eg cover letters, business letters) and e-mails. But it does depend on what kind of letter I'm sending and who I'm sending it to.
p.s. for a quick canvas, you could have used a poll with 3 answers: Yes, No, Sometimes.