Does the CRM installing user need to be a member of sysadmin on SQL Server?
Some companies keep tight control on which accounts can be a member of the sysadmin fixed server role in SQL Server. When installing CRM, although the installing user does not need to be a domain admin, the user does need to have certain administrativwe rights and this includes being a member of the sysadmin role on the SQL Server as per this article https://technet.microsoft.com/en-us/library/hh699825.aspx under the section Microsoft Dynamics CRM Server setup.
Although the CRM deployment guide does not explicitly state that the installing user can be removed from the sysadmin role after setup is complete, this article http://crmtipoftheday.com/2014/04/04/tip-105-your-tip-is-here/ from the CRM Tipsters suggests that it can removed.
However, the user will need to be added back to the sysadmin role to perform an upgrade of the CRM version (e.g. from 2015 to 2016) as per this article https://technet.microsoft.com/en-us/library/hh699747.aspx and this probably also applies when installing an update/update rollup/service pack (or whatever the latest name for an update between major versions is called).