Bill,
Great question. I'm in the same boat. I just joined here in July and love all this great info and discussion found on this sight, and to see all the great cards fellow members have is a plus! I'm currently at 51 and I have asked myself this same question.
I've tried to stay with PSA 3-4 with the HOFers at 4 as financially possible. I've got 10 HOFers. I was going to build my set with a rare back run in twined into it but that makes it to expensive and pulls down set GPA. So what I'm trying to do to not consider backs for my set is to do the back run in SGC holders. Cards in SGC holders look great!
I have a bought a few 2's but they have great color and centering! But this is usually to get a fix!
I have found myself bidding on HOFers and tougher cards lately unless the common is at a very fair price. Tough for me to be patient but when I am I love the rewarding feeling.
Luis