You know, this is one thing NEVER mentioned in "lock-out' discussions. The same point also applies to single phase, multi-wire circuits- that is, where two 'legs' share a neutral!
I found this out the hard way...when a money changer was momentarily supplied with 220v...I really loved having to buy a $600 power supply (as part of a $100 add-a-receptacle job)!
An added clarification....if you are "downstream" of any point where the other circuits might be, then there is no need to isolate them as well. But if...as is often the cas...you have all phases present in the run of lights, they all ought to be killed.
[This message has been edited by renosteinke (edited 02-15-2006).]