That's an interesting question. I have a few other friends with chickens and it hasn't been an issue. I haven't intentionally done anything to increase my biosecurity, but it might help that my "backyard" shoes that I am usually wearing around my own chickens are different than the shoes I wear...
It's definitely a tough decision! In my experience, skittish pullets grow up to be skittish hens. Picking the friendlier pullets is one way to do it. I also sometimes make decisions based on their various endearing quirks.
When I raise chicks I usually share with a few chicken-keeping friends...