If PostgreSQL is also shut down and you dont start the backup before its completely stopped it should be ok. You might need to restore to the same version of PostgreSQL and make sure it is setup the same way. If you dump the data, it is safer, both that you get a known good state, and that it can be restored to any new database. Grabbing the files as you suggest should be ok at least 90 percent of the time. But why risk it?