Last time I checked, there were unfortunately no options to opt out from certain or any listings of your friends' Whatsapp. What is a bit disturbing is that while being a very useful messaging tool, Whatsapp has potential privacy holes which I am not exactly sure about.
Here is a scenario - I have 600 friends on Facebook - I have synced my iPhone contacts and Facebook through the Facebook app. I have also synced my iPhone contacts to my Gmail where I have a minimum of 320 more or less separate contacts.
Now Whatsapp has brough my phone number exposed to all these contacts at the same time. And trust me, I have never shared my cell number with that many people as this is more or less of a private detail.
I just don't know how to prevent certain people from my various contact circles from automatically finding me on Whatsapp and learning about my cellphone number. I cannot just delete the contacts I don't wish to have me on Whatsapp from my iPhone.