From my personal experience, run it if the fragmentation levels are high, your analysis would advice you on whether you need to or not. If you run it regularly esp when there is enough free space, it wont take long, else if you do it like once a year ( as i did ) it will take hours. what i have understood is that it doesnt do any harm (except if the power goes off while its running) since the same amount of wear and tear would be inflicted on a highly fragmented drive if you were to read and write files off it.