uh, it depends.
But for a drum machine, I would assume that you would want to kind of replicate a realistic drum sound. So I would pan slightly to the right and left, because if you're listening to someone play the drums in real life, you wouldn't have half the drums to your immediate left and half to your immediate right, they would all be in front of you, with the snare and hi-hat slightly to the right and the floor tom slightly to the left (or flip-flopped if you want to have it from the drummer's perspective).
Of course, if you're not worried about it sounding "real," then just do what sounds good to you.