With power left on two other phases sharing the neutral, if you lifted that neutral at some point you could end up with voltage on one side via the loads.
Not only would this represent a shock-risk to you, it could also unbalance the voltages on those other loads. Given a large load on one phase and a small load on the other, you could see the voltage across the latter rise to almost the phase-to-phase level.
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).]