That all depends on what you prefer.
If you have a custom domain registered elsewhere and you purchased one of our paid plans, you can use that domain as your primary address for your WordPress.com site.
You can either map it – your domain will stay with the current registrar and you’ll need to go to their site to manage anything domain related.
Or you can transfer the domain to WordPress.com and we’ll be your new registrar. This way you’ll manage everything from one place and you’ll be able to reach support with any domain related-issues :)