It occurred to me that Sylvain Briole wrote in gmane.os.netbsd.general:
> It works perfectly, but in order to automatize installations, I would like to
> know if it is absolutely necessary to reboot in single-user mode before first
> reboot in multi-user mode, and when yes, why?

It isn't - at least, not for the sake of editing rc.conf. (You should never
edit /etc/defaults/rc.conf, if you ask me). sysinst doesn't do it, so why
should you? 

> When single-user mode not absolutely necessary, is it possible to set (in
> /etc/rc.conf) :
>
> rc_configured=YES
> sshd=YES
> dhclient=YES

You probably need a dhclient_flags, containing your ethernet interface,
but that should work.

> And also install (in chroot or something like this) some packages like bash
> for example, and then finally reboot?

Probably..