Before you start, print /etc/passwd and /etc/group. :-)
When forced to create a user during installation, I create user x with passwd
x, and the first thing I do on first boot is login as root and delete user x.
I then install mc and do my "tidy up" before creating the group(s) I want and
then use a script to create users, followed by setting users' initial passwds.
/etc/passwd and /etc/group are still the last word in identities in these latter days of password shadowing and gawd alone knows what else?
(My own research suggests the answer is YES for what users are on the system and NO for passwords. I'm not doing anything funky like using LDAP or anything for passwords, just what comes out of the box when you install Debian)
Mark