over working a muscle will slow growth. many people get their tricep workouts done via bench presses and dips, and then just toss in a single isolation exercise (i.e. pressdowns) as a final 'pump' move, knowing the meat & potatoes of their triceps training occured during compound chest exercises.
I'm a fan of close grip bench, skull crushers (a.k.a. french press) and cable press downs as my specific tricep moves, which I typcially only do a couple close grip bench sets, and 1-2 pressdown sets, usually on back or shoulder day, as I find my triceps grow better if they are secondary muscles on chest day, and then hit em directly a little a couple days later. But that's just me.