I do them a variety of ways. I assume you mean seated.
I generally always tap my shoulders with them, and as for the lockout, I have had success both ways, locking each rep out fully as well as taking them close to lockout. I can tell you right now, time under tension be damned, taking each rep all the way to lockout is significantly harder, especially if high reps are done and you do them strict standing.
Doing them standing strict will have a profound effect on how they feel and what they do for you. You don't have the back support to assist the initial accelleration off the shoulders, so you have to keep your core tight, and press slow enough to remain in control of the dumbbells. This is a very humbling experience for most people who are used to doing them seated.
I wrote a quick article on high rep strict standing dumbbell work, let me know if you're interested and I'll post it.