When it comes to step climbs I usually wait until the optimum is at least 100' above the half way altitude. This means that in CRZ F330 with 2000' steps, I will climb to F350, all things being equal, when OPT shows F341. That allows for a bit of error in actual gross weight. Remember that anytime you are not at OPT level you lose fuel but you can claw a bit back by reducing speed when below but when above you just lose. Tactical thinking may, of course, make an early climb the best bet overall. Break even winds too need to be considered.