[CAUTION: I am not a teacher:take the advice and or corrections offered in this post at your own risk.
If you doubt the information, please get a qualified opinion from one of the teachers on these forums.]
Depends on my mood, with whom I'm talking, and the conversation. I think this varies greatly from person to person as well. I usually add idiom when I'm trying to be funny or euphemistic (say something that might be offensive in a nice way).
I had an older friend though, who would use idioms so liberally that his speech was often more idiom than direct expression. It was really funny sometimes and slightly confusing at others.