Whatever feels best, and for whatever position you in which usually have your hands. That's the simple answer. Riders who ride on the hoods a lot will probably have theirs set differently than riders who use the drops a lot -- or maybe not.

If you're having trouble reaching the brakes while in the drops, you might simply be expecting the wrong thing from the position you're using. When your hands are more, I guess, "straight down", and near the ends of the tubing, the expectation is that you're just cruising quickly, and not necessarily descending or sprinting for top speed -- so it's not a priority to have the brake levers at your fingertips. But if you crouch a little lower, get that flat back, and reach your hands deeper into the handlebar's hooks, you should be able to brake and shift quite easily -- which is more of what you need when you're really cranking.

When you really want to get into handlebar positioning, take off the bartape and ride around for a while with a wrench or two in your pocket -- one to adjust the bar and the other to adjust the placement of the shifters. Experiment with that until you find a combination that you like, then tape up the bars again.