For one, just adding a rep or two really isn't going to do much of anything, though it can't hurt, as long as you don't go to failure. On the other hand, the hammer curl on an incline, (if that's what you are doing), is more for stretching the muscle under load, so it's not as important to keep increasing the weight. Jules mentions that somewhere in this thread.
Frankly, I'm not even sure it's worth doing an isolation movement like that during the 15's if you're hitting your bi's with other movements.
Frankly, I'm not even sure it's worth doing an isolation movement like that during the 15's if you're hitting your bi's with other movements.