One of the powerful feature (but look simple) in Salesforce is Sharing. Yes, it just sharing and many admin/consultant not really care on "Sharing Settings". But, you need to configure it correctly to make sure your sales rep are not fighting.
Based on experience, this blog will share a little on sharing for Contact, Opportunity, and Case.
If you go to Account Sharing Rules, you will find Contact, Opportunity, Case sharing.
Then if you go to Contact, you can find Contact Sharing Rules, the same for Opportunity and Case.
So why double? Is it the same thing? Yes and No.
For example Contact.
Yes, when Contact owner = Account owner, so it will be same.
No, when Contact owner <> Account owner, why?
In Account sharing rule, if the Account owned by Mike, and contact tagged to that Account is owned by Bruce, the sharing rule in Contact will not effect for Bruce.
But, if the sharing rule is in Contact, no matter who owned the Account, the sharing rule in Contact will ALWAYS effect.
So, which one is the best? No right answer, it will be based on your business requirements.
Hope this make sense and help.