I agree. Robin joined at the end of the crocodile arc. Because she didn't play the crew and faked to be a part of them.
You can see that because she left them behind in the CP9 arc because she cared so much about them and wanted to keep them safe. That shows that she was a true SH by that time.
Nami is much more questionable. She joined at the end of the Buggy mini arc, but she only pretended to do so.
Sure, she was having a good time with them, liked them and wanted to stay, but she betrayed and left them of her own free will.
There were of course reasons to do it, but still.
And than she joined at the end of the Arlong arc. But even in her case I'd say she joined at the Buggy arc.
So you should forget about Robin and discuss Nami's case (although thats unnecessary too)
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"