Two separate questions:
1) Who should own databases?
It does not matter if the Server Login that owns a database is disabled or enabled. sa is a good default choice to own all databases, regardless of whether the Login is enabled as the Login's status will not affect database operations.
What does matter is whether the Server Login that owns a database has been deleted from the instance, or if owned a domain account that entered the instance via an Active Directory Group whether the account has been deleted from Active Directory. In the case when the Login or underlying account is deleted, if you're leveraging cross-database ownership chaining you might start to see problems. The worst issue the average installation might encounter if a Login or Account that owns a database is removed is you might see an error when right-clicking on the database in SSMS Object Explorer and selecting Properties.
2) Who should own Agent Jobs?
Same story here for the most part. sa is also a good default choice to own all Jobs even if the Login is disabled. When sa owns a job, or any Login in the sysadmin Role for that matter, the job will execute under the context of the SQL Server Agent Service account.
One of the things I want to know when I takeover a new instance, and also check periodically through either Policy Based Management or a simple job that emails me a report, is which databases and jobs exist in the environment that are not owned by sa, especially the ones owned by Server Logins or Active Directory accounts that no longer exist. I then explore whether I can standardize those databases and jobs to be owned by sa.
There are no special teachers of virtue, because virtue is taught by the whole community. --Plato