It does depend on the age of the machine. Anything newer than around 1998 (I think) it should be between 320 and 340. This depends on the incoming supply voltage and there are transformer taps that can be moved for different supply voltages at the bottom of the control cabinet.
The DC voltage does flicker around a bit and when the machine is accelerating it will drop quite a lot or deccelerating it will go high. If it drops below around 280 volts during acceleration the machine will alarm, there is a parameter and time limit for low voltage. During deccelration if it goes to high because the regen resistors have not kicked in or something it will also alarm.
Running with the DC voltage always near the high limit might give regen overheat alarms if the machine is doing a lot of starting and stopping; running with the voltage low may give spindle overheat alarms under the same conditions.
If your incoming supply is a bit limited in its peak capacity the low voltage alarm may occur frequently. This will happen especially if you overlap spindle acceleration to maximum speed at the same time all the axes are run from the tool change position to the start point for a cycle; this puts the maximum draw on the DC bus because there are four motors pulling maximum power. If this happens the fix is to separate spindle acceleration from axis movement or drop the spindle acceleration slightly; there is a parameter for this.