The Apply Data Item Access setting only impacts Scheduled workflows which may require a user to be logged into the windows Application in order to run.
Selecting the Data Item Access check box means that a Scheduled workflow will only run if the Data Records are Accessible to the user that is logged into the Windows Application at the time of running.
Selecting the check box will not vary user’s ability to view records it only restricts when the workflow might run.
Unless there is a specific reason for restricting a Scheduled workflow only run when an users’ data item Access allows then leave the Apply Data Item Access box clear