Quite often the magnetisation occurs during transport or storage and therefore the supplier is normally blameless.
Magnetisation can occur and does have an impact on weldability as the arc can veer away from one weld face to the other. I've seen welders wrap their power leads round the pipe in the past to act as a de-magnetisation input, but for more severe actions, you would need to bring in de-gausing equipment. For new pipes it's fairly rare, but for tie-in to existing pipe is more common, especically when the pipe has been internally inspected by magnetic flux leakage intelligent tools.
As king nero says, is this an actual problem or are you just trying to get procedures in place to deal with a potential problem?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way