Okay, so we've established that
Dude, you shouldn't be walking around with a fork in your mouth!
... is an idiom. We've also established that it is in the same category as
Dude, you're playing with fire.
More details (from the comments above):
Hold, have, keep - all those verbs apply, and "having a fork sticking out of your mouth" works too. That said, the idiom is still puzzling unless you explain its meaning to the person you're talking (or writing) to. My bet is it'll still sound a bit puzzling even after you've explained it, and here's why:
Different cultures have different versions of sadistic cruelty, and while "Go play in traffic" is a cruel joke that pretty much anyone can understand, their background notwithstanding, traditional idioms are harder to grasp at once because they reveal to us new shades of conversational sadism we haven't been exposed to previously. Adjustments need to be made, the unpleasant sharpness and the implications of the image the idiom evokes need time to wear off a bit before the idiom becomes just another run-of-the-mill conversational device.
I suggest you use something less culture-specific, more universal. Make one up if you have to. It's easier than some people think. "You shouldn't go jogging in a mine field." "You shouldn't play "fetch" with a crocodile." See? It's pretty easy.