If you are looking to target your rear delts specifically i would recommend blasting them with a few exercises first, at the very beginning of your workout.
I do think it's important to train the rear delts, I wouldn't recommend training them at the very beginning of your shoulder workout since you'd be losing intensity on the rest of your exercises. It would be like starting a chest routine with flyes, or a back routine with machine pullovers, but probably even less efficient. This is my opinion though, so of course do whatever you think would suit your training best. If you get good results by doing this, by all means keep doing it. I've just found that in my own experiences, it's usually best to start your routine (after warmups) with your heavier, compound lifts, then add in some isolation work afterward. The point being to focus the bulk of your efforts into the lifts that will give you the most response. As far as some good exercises to start your shoulder routine with: military press, upright rows, clean and press, Arnold presses and DB shoulder presses. There are others, but those are some of the most popular ones. You can pretty much bet that whatever exercises you use, you're going to be hitting your front deltoids a lot, so it's fun to try and figure out routines that will also hit the side and rear heads to a greater degree as well. Of course no one head of the deltoid works in complete isolation, but by choosing the right exercises you can put more emphasis on each head of the deltoids. If you're wanting to target your traps on shoulders day, why not do some upright rows since they work the front deltoids and traps to a great degree. Then if you wanted to add intensity to your trap workout, you could add in some shrugs. A really intense trap workout might be a superset of upright rows followed by shrugs. So what I'm trying to get at is to think about which muscles you want to do the most work and craft your routine around that idea. By doing this you'll have a much easier time at keeping your physique balanced and in proportion. Keep in mind that my philosophies are geared towards creating aesthetic, balanced physiques, rather than just sheer mass and brute strength.
Something I really like doing for shoulders is a superset of Arnold presses followed by rear delt lateral raises. Or I suppose you could swap rear laterals for any other rear delt exercise. Since Arnold presses really hit the side and front delts hard, it's always fun to add in a rear delt exercise right afterward. In general, I'm fond of doing rear delt work as part of a superset after a compound lift.