The 1951-S is a bit tough to get without going to the -65 level or higher. My problem with the BTW's is I can't find ANY 1949's, (P), D or S. At least not in a grade I want, -64 or -63.
You are farther along than I am. Kudos. Just started recently on my early commemorative set and I have a long way to go.
I'm going to have to agree with the "send it back" crowd. I finished the 50-coin set a couple years ago, and I went for mostly the "bang for the buck" grades, which are the highest grades on the flat part of the pricing curve, and then looked for ones I liked. I probably made a few mistakes along the way, but they were cheaper ones (one reason I advocate buying key dates later when building a set). I might do a complete Oregon Trail and/or Texas set, but probably won't do the complete 144-coin set. Too many other irons in the fire right now.