I'm not trying to break out some revolutionary new training idea by any means, just wanted to share an idea. I think, unless you're with the advantage of steroids, that you should not ever train your delts specifically.
There are reasons for this, I won't just spit blindly into the wind. Take the view that you already hit your delts doing other exercises. Bench press and most pulling motions will work the front and rear delts respectively. But it goes well past that. For your arms to move in any direction requires the use of your delts to some degree. To simply stand around with your arms at your sides and dumbbells in each hand will force your middle delt into action. And how many training days do you hold weight, or have to heft a dumbbell onto your lap before laying back and benching, for example?
The idea is that you are already working your delts well just by doing other lifts, carrying weight, etc. So don't specifically train them because they're already on the verge of overtraining as it is. Instead, where you would normally insert shoulder work into your routine, give some extra loving to the parts that really want and need it. Like calves.