I can confirm that it is safe to use also the workspace.prm file and you are right, in such a case, you don't need to link it to the graph/subgraph each time as the workspace.prm file is already linked to each job.
Additional note: You have to add your parameter to the workspace.prm file in the root directory if you want it to be "global". I mean, the main workspace.prm file is in the Navigator pane at the bottom of the project's directories and files list. You can double-iclick the file there and edit as usual. This way the parameter will be shared among all graphs. On the contrary, if you add the parameter within a single graph by double-clicking the workspace.prm file in graph's Outline pane, you will see it in that single graph only.
PercyJin, if you decide to keep your parameters in a separate file (as I suggested originally), please follow the instructions on the following page:http://doc.cloveretl.com/documentation/UserGuide/index.jsp?topic=/com.cloveretl.gui.docs/docs/index.html
On the above-mentioned page, there is also described how to "Link" the PRM file to each graph. You can skip this point by drag and drop the file from Navigator pane to the Outline of the graph in question. If you have any difficulties with the process, please let me know what exactly do you struggle with and I'll help you through.
Thank you both and have a nice day,