Adding Users Command line

["Bret Stern" <bret_stern machinemanagement com>]

When adding users from the (command line)

useradd 

has an option -r which assigns a specific user id.

Is it common to explicitly assign a user id?

What happens if I don't specifically assign a user id?

I'm assuming two users cannot have the same user id,
so..how would you know the user id's of all your users?

Thanks