Everyone is going to have their own opinions and different ways they do things, but here's mine
I like to get an xray around day 58, give or take. The reason being is so I know how many pups she is carring. That way you will know what to expect and you will know when she is finished.
Also, from the xray , the vet may be able to tell how big the pups are and if she should have problems in deliverying them. My vet says that the xray is not a real accurate way to see the size but it still gives an idea.
So that is how and why I do my xrays.
I hope with helps!