I like using a "3 down/5 up" rep for most exercises (though shoulders seem to like 4/4 better.) The "2 down/6 up" rep that is used makes the 2 down seem too fast to me, so I like to increase it.
I've also used a 3 up/5 down rep (to stress the negative portion of the move).
Often, I mix up rep patterns: 3 down/5 up; 5 down/3 up; 6 down/2 up.
I think that mixing things up by varying the rep count would give the best results because it doesn't allow your muscles to adapt to the workkout as easily. As long as you can "keep the muscles guessing," they are working harder and can't find a more efficient (and thus less effective) way of doing the workout.
I like to mix up reps on other workouts as well. Usually, if there is a 2/2 count, I will often change it to 1 up (explosive), 3 down (controlled).