Blake,

My development cycle on a new custom workflow activity is to build the new version of the DLL, stop the workflow service, copy the new DLL over the old DLL, then restart the workflow service.  Workflow then loads the new version of the DLL and you can proceed to test against it.

The only thing to look out for is if you add or remove any properties from your custom workflow activity between builds.  The designer will throw an error if you open a published workflow that contains a reference to the old activity because it can't deserialize the properties correctly.  The workflow will open but you will have to delete the old activity and insert a new copy of the activity to allow the designer to properly render the new properties.

You are welcome to PM me if you have specific questions.