I am aware that horses that are not overo can carry the gene. I know that my horse does not have overo pattern in her back ground for the 7 generations I can trace, not sure if that makes a difference or not.
The stallion however has had some rather extensive genetic testing done. So I can ask the owner if she knows. I also know that it is not very common to have this issue if you are breeding two horses and neither of them have the overo pattern. So I may have this testing done later on Magic if we decide to have another foal with a paint stallion I don't believe at this time it is necessary. Both horse have to be carriers in order to have a lethal white foal. I just read that even some overo horses are not carriers. It is a good test to have done, and if our vet had recommended it we would have had it done.
Any horse being bred that is of any breed that has frame (or any horse of unknown breeding) should be tested before being bred as a matter of course. It doesn't matter how much white they have, or how much white they don't have. If the potential is there, a simple $25 test can save the 25% chance of a foal that is born to die a slow, painful death within 72 hours of birth. Any argument against that is absurd and irresponsible. If you plan to breed your mare again, to ANY stallion that could possibly carry frame (not just a Paint FYI) then it is your responsibility as her owner to test her.