If you're bouncing between multiple PM's, then yeah it makes sense to have them calibrated among themselves. But if you've got only one PM, what difference does it make as long as it's consistent? A 1w gain is a 1w gain no matter what.
It's like the spinal tap joke about "this amp goes to 11" -- it's not louder because the dial reads 11, it's louder because it puts out more decibels. It makes no difference what the scale on the dial reads, the thing that matters is the volume coming out.