The determination of whether you will associate Sales and Customer Service objects with Contact and/or Account objects depends on whether you are conducting business with other businesses (B2B; Business to Business), or you are conducting business with individual consumers (B2C; Business to Consumer). Some companies serve both markets and will have a mix of B2B and B2C. Many of the modeling decisions you make in setting up your system will be driven by this determination.
Based on your business model, you will want to establish guidelines around how users attach objects to each other. In a straight B2B model, it is preferable to have all Contacts, Opportunities, Service Contracts, and Service Cases attached directly to (that is, parented by) Account objects. In a B2C model, you will want Contacts to act as the parents of all these objects. Of course, this can vary based on the details of how you do business, but the concept of consistency should be introduced and enforced through training and business rules.
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