Neanderthal no more is great but it takes a lot of experience to do these kind of checks. It is almost guaranteed that there are some deviations from a "good" posture since a good posture is just a mean.
Since you found a lot of spots on the pecs then I'd assume some more dedicated pec and pec minor stretching would be advisable. Check the lats as well. From what your saying though I'm pretty sure that the addition of some pec and lat stretches would be useful but I advize that you do the massage for a while FIRST until you have worked away a lot of the pain and then add stretches. Doing the stretches right away is not automatically a bad thing but I believe that improving the tissue quality through the rolling and massage first will make the stretching much more successful and quicker.
This is purely anecdotal but I had the same pain between my shoulder blades and it definitely coincided with a whole lot of hot spots in the pecs. Now I am hard pressed to find a hot spot anywhere in my chest since foam rolling, massaging, and stretching and the pain between my shoulder blades is gone. But the first thing I did (quite a while back) was to try to stretch the actual rhomboid/mid trap area and that was definitely the WRONG approach. Just made it worse. And I have found that to be the case more often than not.
Do that after you work upper body. And if you are dong rows you may want to stretch the pecs before hand as long as you don't plan on needing them again. It could help you find a bit more range of motion on rows. If it works than do it, if it doesn't don't worry with it and stick with the foam rolling and/or tennis ball before you workout (short period) and you can do a longer period after you work out or at any other time.
I'd still like to see you in action for myself. Not to discount Joe :)
I know you're getting the trigger point manual so that should allow you to figure out how you want to handle the self-massage and the tennis ball can go hand in hand with that. Foam rolling is more of a general thing but of course with certain muscles you can pause on the bad spots. Well..actually, you shouldn't really "pause" but more go back in forth with short rolls over the bad spot.
You may want to also look at Eric Cressey's Shoulder Savers. I know Joe is familiar with all this stuff.
I'll shoot you a vid with some good info on the pec stretching.
To stretch the lats (after working out):
Bend at the hips and grab on to any upright thing that is stable. Keep the back set. Then pull against the object and use your hips to change the angle until you feel a good stretch in the lats. You can move it around a bit to move the stetch. You just need to experiment with it.