Starting from Telerik Reporting Q1 2014 release both the Silverlight and WPF report viewers support only implicit styling, i.e. style without the x:Key attribute. Implicit styling has several advantages over the StyleManager approach, like smaller assemblies and full Expression Blend support - for more information please check XAML Theming – Implicit Styles versus StyleManager blog post. Additionally you can take a look at Setting a Theme (Using Implicit Styles) help article as it contains important information about how to properly set the implicit theme for the ReportViewer control.
If you are starting a new project please have in mind that using implicit styling for both Telerik UI controls and Telerik Reporting viewers is the recommended approach.
However if you already have a project that uses the StyleManager it is not mandatory to migrate it to implicit styling for all of the controls. The implicit theme will be needed only for the ReportViewer utilized in your application. This means that the assemblies referenced in your application can still be the ones with the embedded XAML. However, in order for the ReportViewer to work, all the XAML files for the theme have to be included, as described in The WPF/Silverlight report viewer is blank after upgrading KB article. The other approach is to include only Telerik.ReportViewer.Wpf.xaml/Telerik.ReportViewer.Silverlight.xaml file when the theme is set globally as described in Setting Application-Wide Built-In Theme in the Code-Behind paragraph. Otherwise the ReportViewer will use the correct theme, but the Telerik UI controls in it will always use the default Office Black theme.
The negative effect is when you want to use one style for the ReportViewer and another for the rest of the controls. In this case the implicit styling will take precedence and override the style set by the StyleManager. That means that all controls included in the XAML files needed for the Report Viewer will be displayed with the ReportViewer's style and the rest of the controls will use the StyleManager style.
In case you have any specific scenarios we would be glad if you share them with us so that we can better understand and help you with your goals.