Well, you're right, bfold. Going all the way up probably is better. I just like to save my tris for other motions, and I feel that most of the delt contraction is in the bottom half of the motion. Just personal preference, I suppose.
I feel the lockout, especially with dbells, in my delts. Kinda like doing a flye for the chest. I touch the dbells together at the top...which squeezes (contracts) the delts...
I've only been doing dbell overheads for about 3 weeks though...