The user should be dbo when you install Sitefinity or you perform an update.
db_datareader and db_datawriter are required. The db_datawriter role allows its members to perform modification of existing data and to insert new data. The members can execute the INSERT, UPDATE, and, DELETE statements against the database objects in a database.
Modules like Forms creates database tables
and if you do not have permissions you will get an error. All custom fields creates columns and new records in existing tables.
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