Skut,
When I was in load control we were governed by our Ops Manual for standard weights. This was then approved by FAA.
The standard weight used also dependant on region as some flights on average heavier than US domestic flights. Every now and then the company would do a baggage audit on particular flights to maintain correct standard weights used.
All flights from US and the return used same weights. Europe for example was 30lbs per bag. US domestic I believe is 25lbs.