TDE prep: key/certificate backup for restores
If you want to restore on a different server, you should be able to do so with the certificate, private key and database backup file(s). 

When a certificate is created in any database in SQL Server, it is part of an encryption hierarchy. The certificate in the master database itself only contains a public key, which needs no protection, however, the master database will also contain a separate but mathematically related private key which does need to be protected. The method of protecting the private key is to encrypt it using the database master key you created in the master database prior to creating the certificate. The next layer in the encryption hierarchy is that the DMK is encrypted by the service master key. There is only one SMK on the system and it is in the master database. 

Even though you don't need the DMK and SMK to restore an encrypted backup to another server, I would back up these two keys anyway as it makes recovery much more flexible. 

When you restore the backup encryption certificate to the master database, what happens behind the scenes is that the private key is read from the file, decrypted using the password you provide in the restore command, then encrypted using the database master key and saved. As you know, the private key from the certificate can then be used to decrypt the Database Encryption Key in the backup file and successfully restore the database. 

I don't have a specific recommendation for storing the certificate and key backup files, but they do need to be available to you and whoever does the disaster recovery in your organization.