Lethal White Syndrome is primarily a Paint syndrome and pretty much is an "overo" thing. Being that it is rare to have a horse that is registered both in AQHA and APHA, and you are looking strickly at quarters...I wouldn't worry about that one.
Poco Bueno's line is known for HERDA and you can read more about it here HERDA
And one that is not traced by line is GBED which you can read about here GBED
But I think you only have to worry about this one if you are breeding as horses born with the actual disease die shortly after birth...you just want to make sure that your mare or stallion does not carry the gene because their foals will be affected.