Using a good registrar is key but you also want to separate host from registrar because you want to keep your content.

It is a much more common case to change hosts than it is to change registrars.

But even if you lose your domain name somehow through a bad registrar your content can also hold a lot of value and though you would take a big hit by changing domain names for a lot of sites (like blogs and other content heavy types) content is key.

See the recent incident with cyanogenmod the move to .org went rather well until the .com was given back and not having a separate host can make it so you don't even need to restore from backups/setup a new environment.