We tend to use our own perosnal ids for creating scripts, or individual testing. We user common/shared ids to run scripts once done or in load testing. But this all depends on the testing being done as well. If each tester needs to have a specific profile built to do their testing then you do not want someone else going in and changing it to do their testing. You have now created a conflict in testing which would have been avoided if each tester used unique ids.