Hello Roger,
it may be a 'standard' modification but it is uneccessary and from an engineering viewpoint ineffective.
The tapered bolt needs to be fitted properly and should seat in the cross arm. If that is not done correctly, an additional parallel pin will not be effective. The whole reason the pin,as designed, is tapered is to completely eliminate any relative movement, this is impossible with a parallel pin.
Do the job properly and there will be no problems. I have had an instance of tapered pin bottoming on the head, this will not work and I had to relieve the head such that the pin sat correctly in the tapered bore.
Alec