I think you have been given some very good advice. One thing that I do, is mark the seam of each block on the border. So if my blocks are supposed to finish at 12 inches, I will have marks at 12 1/4, 12, 12 and so on until the last at 12 1/4. If the blocks have a lot of HST's I will even go so far as to mark the center of each block. I find it is the best way for me.
You have been talking a lot about borders, but what about your binding? I made a baby quilt that ended up with a tsunami of waves on the borders. I was soooo frustrated because it was a gift. I ended up taking off the binding and letting the quilt just sit for a few days. I then re-pinned my binding on before sewing, and had a whopping 5 inches extra!! Guess I was stretching the body of the quilt while I was sewing on the binding.