Pretty much what the titile of this post says - do you use funny voices, accents and impressions during the game sessions that your run?
I know I do. I've been doing so since my teens, and I think my skill with such has only gotten better with age. For me it adds flavor, and makes NPC more memorable, especially if you can keep the voice consistant between sessions.
If you do use voices, are there any techniques that you use to make is special?