By default, Salesforce allow Contact do NOT link to Account, unless you set Account as required in Contact Page Layout or Validation Rule.
Although Account sharing rule is Public (Read Only or Read/Write), for a Contact without linked to Account, regardless what is the Contact sharing rule, it is considered as Private contact, meaning: only contact owner, user with View All Data and View All [on Contact object] permission able to see the Contact, while all other users will not able to see the contact (even if the user copy and paste Contact URL, he will get error message: Insufficient Privileges), and the contact will also not shown in the search result and report.
Sharing rules and workflow rules do not apply to private contact, you also cannot manually share and enable self-service for those contacts, both buttons will not be visible.
So in summary, it is not a good idea to leave Contact without Account. You can request Salesforce Support to enable Person Account for such scenario. Using an account as place holder is not a good idea as well, as the number of contacts linked to that place holder account growth, it will cause data skew issue.
If you enable Person Account, contact sharing in Organization-Wide Default for Contact will be automatically change to Controlled by Parent and this is not editable. So, if you just enable Person Account, Organization-Wide Default for Contact will be changed to Controlled by Parent, this will remove all existing contact sharing rules and manually shared contacts.
- How can I create a report showing accounts without contacts?
- Avoid Account Data Skew for Peak Performance