What does automatic login do?
Instead of waiting for a user to enter their name and password, Windows uses the credentials you enter with Autologon, which are encrypted in the Registry, to log on the specified user automatically. Autologon is easy enough to use. Just run autologon.exe, fill in the dialog, and hit Enable.
Should I login user after registration?
It can be safe to auto login if the user already has an active session as the correct user during the confirmation step. If you think about it, it's not actually "automatically logging them in" but simply keeping them logged in as they was before. During all that time, there was no reason to end the session.