I will make sure if I want to make only website submission's DLL or make another DLLs using the same interface. In the first case make sense to write the check at application level because you'll produce less code at DLL level. But if you want to write not only website submission plug-in I will write the check at plug-in level and will have the exception handling at application level as you write. Generally whatever you choose it should be clear and documented for plug-in programming purposes.