Here is the deal. I ask this same question when I first started using Hitec items. I ask here and also at Hitec. I got the same basic answers. I can guarantee you this works...... at least it has for me. Beg, borrow, or buy, the type of arm you want to use. Get a BUNCH of them. Now start picking up one at a time and place it on the shaft. You WILL find that some will be dead on, and some will be as you said, off center a hair.
I have done this with their heavy red arms, and 4 point arms in white nylon. I am sure it will work on all their servo arms. I found that about 1 in 4 seemed were good. Of course that is just the group of parts I had. It could be better or worse..... but you WILL find some good ones. I have almost all Hitec Rx and servos, and am very happy with their products. But that is the one thing that irks me!! I have never had that problem with Futaba. .....lownslo...