Our specs call for RT prior to PWHT to detect volumetric defects unless it is a machine or MIG weld where we also require UT prior to PWHT. After PWHT we normally do UT checks to detect any cracking and usually do a few hardness checks. The hardness checks are just a test to verify that there were no material mix ups. The hardness checks have been mostly eliminated due to better PMI equipment.
In my 42 years I've never seen any problems with P-1 material during and after PWHT.
As said above by weldtek , RT shall be done after PWHT for high alloy steels per UHA-33. This is because a delayed cracking or post heat treatment cracking may happened in or adjacent to welds after PWHT. This is not applicable to UCS steels. Common sense: you always need to do RT before PWHT to repair defects if any. Section VIII Div. 1 does not require RT after PWHT for carbon or low alloy steels.