The rule is very clear.
Look Out Sir attempts are made when the wound is allocated to a model.
In single save units you roll to wound, take as many saves as required, and then allocate them in turn.
If a character is allocated a wound at this stage then a LOS roll may be made and either the character or another model takes the wound immediately since a save has already been taken.
In multi save units the wound is allocated to a character singly since he has terminator armour on (for example). You must then immediately make the choice to attempt for LOS since the wound has just been allocated.
Either you can attempt to use the better armour save of the character, but risk him taking a wound, or you can LOS and allocate the wound to a grunt using their worse save from the off.
You cannot take the 2+ character save and then choose to LOS it if it fails, since that would be against the rule that LOS is attempted immediately after the wound was allocated.