Mostly, the reason for knowing this number precisely is because after the angle of the bed is known, this information is used to correct the height of the Nozzle. The further the probe is from the Nozzle, the more the height will have to be corrected. In practice, what people do is look at the first layer going down and decide if the nozzle is too high or too low and adjust the Z-Probe-Offset to make the right thing happen. You numbers are accurate enough to get good results.

My current numbers are:
#define X_PROBE_OFFSET_FROM_EXTRUDER 18
#define Y_PROBE_OFFSET_FROM_EXTRUDER 17
#define Z_PROBE_OFFSET_FROM_EXTRUDER -3.85 //-4.20 prior to declogging nozzle on 07-07-2014

And notice the comment on the Z Axis. After taking apart and reassembling my extruder the number moved .5 mm. That number was not determined by measuring things. It got to where it is by watching the first layer and moving that number up and down until I liked how it was going down.