I get this question a lot when working on the SQL Server Reporting Services platform. I think from experience I have boiled this down to some direct (albeit high level) guidance to answer this question.
Use SSRS in SharePoint integrated Mode when….. (We already love SharePoint at my organization)
If your organization or company is already taking advantage of SharePoint, then plan on installing your SQL Reporting Services instance in SharePoint integrated mode. The reason? If your organization recognizes SharePoint as a one stop shop for content management, document management, collaboration, team sites, search…..on and on….then the natural place for your report consumers and business analysts to access their reports is within SharePoint.
Use SSRS in SharePoint integrated Mode when…..(We are pretty sure we are going to love SharePoint at my organization)