[supershazwi]

I feel that asking a user for the right username after they sign up wastes even more time when they can settle it all at once at the sign up page... The tips mostly reduce hassle and why cause them more trouble by alerting them that their username is already taken when they felt they're already set to use the site.

[dpcan]

But asking for the username after sign-up gets you the user's email address. If they don't create a username, you can now provide direct support and ask if they need help with the next step in the sign-up process, etc.