Isolation movements are a waste of time unless you're already pretty massive or juicing.
Though I'am a firm believer that people waste far to much time worrying over isolation, isolation shouldn't be totally excluded from someones routine, regardless if they're stacked or on the gear.
As an example, After my ohp heavy work I repeat ohp for another 5x10 sets (at lower percentages) where obviously the muscled used predominantly would be the trapezius, levator scapulae, serratus anterior and rotator cuff...all working together to stabilize the movement, the core & the triceps. I have been doing more direct tricep work (when I say more I mean push downs but I wont keep a log of the work) as to improve my lock out as the work has become more difficult.
Assistance should be aiding the main lift, not detracting from it. I can't remember when I trained any direct bicep work, yet mine still grow. Assistance work should keep you balanced...just give what you do a valid reason, having big biceps isn't a valid reason lol