There was an interesting post on the Reporting Services public newsgroup about an issue with the RS 2005 ASP.NET Report Viewer and custom assemblies. In a nutshell, the issue was that if you want to invoke a custom assembly from the ASP.NET Report Viewer, you need to deploy it to GAC.
Because of a bug in the RTM version, copying custom assemblies to the web app bin folder will not work. This bug will be fixed in a service pack. Currently, for web applications, you have to copy custom assemblies to GAC. This issue doesn't apply to the WinForm version of the Report Viewer. In the case of WinForms apps, custom assemblies can be in the same directory as the application's .EXE file; no need to copy to GAC.
An example of how the WinForm version of the Report Viewer can be configured to work with custom assemblies can be found on the Report Viewer home page. This page is maintained and monitored by the Rajeev Karunakaran, a Program Manager for the Report Viewer controls.