I’ve ofter wondered about this. Turns out it’s a bit of an old wives tale.

Turns out, however, that power surge is so brief that its energy draw doesn’t amount to much: the equivalent of a few seconds or so of normal operation, according to U.S. Department of Energy estimates. In other words, from a strict energy-conservation standpoint, it’s almost always beneficial to shut off fluorescents when leaving the room—the start-up energy is offset by the power saved in even the briefest outages.