Best practice is to register the assemblies to the database.
Plug-ins not-registered in the sandbox can be stored in the Microsoft
Dynamics CRM server's database or the on-disk file system. We strongly
recommend that you store your production-ready plug-ins in the
Microsoft Dynamics CRM database, instead of on-disk. Plug-ins stored
in the database are automatically distributed across multiple
Microsoft Dynamics CRM servers in a data center cluster. On-disk
storage of plug-ins is useful for debugging plug-ins using Microsoft
Visual Studio but is mostly provided for backward compatibility with
callouts. You can debug a plug-in that is stored in the database.
Plug-ins registered in the sandbox must be stored in the database
regardless of the Microsoft Dynamics CRM deployment (on-premises,
IFD/SPLA, or Online).
This excerpt from the MSDN describes the registration possibilities and the paths.
For on-premises or Internet-facing Microsoft Dynamics CRM
installations, when you deploy plug-ins from another computer to the
Microsoft Dynamics CRM server disk (on-disk deployment), the plug-in
assembly must be manually copied to the server before registration.
The assembly must be deployed to the \Program
Files\Microsoft CRM\server\bin\assembly folder on each server where
the plug-in is to execute.
Plug-in registration should be done after the assembly has been copied
to the …\bin\assembly folder on the server to prevent the situation
where a system user causes an event in Microsoft Dynamics CRM to be
raised but the registered plug-in assembly does not yet exist on the
server. For server database deployment, the plug-in assembly is
automatically copied during plug-in registration so that the earlier
situation is not an issue.
Depending on your plug-in’s design, your plug-ins may require other
referenced assemblies to run. Regardless of whether you deploy your
plug-in to the database or disk, if your plug-in requires other
assemblies to run, you must put copies of these assemblies in the
Global Assembly Cache (GAC) on each server where the plug-in is to
execute. This does not apply to a Microsoft Dynamics CRM Online server
because you do not have access to the GAC on that server.
See Register and Deploy Plug-ins
