Thanks for your replies. I will be getting a boxer for companionship, so color is not really an issue for me. The issue is making an informed choice and being a compassionate citizen and a responsible consumer (that sounds sort of cold, but hey, money talks!). I probably will not purchase ANY puppy from that particular breeder as she has not convinced me of the merits of culling.
Here's where I get on my moral high horse ... feel free to correct me if I'm wrong.
I've read that culling originated from when boxers were bred as guard dogs. White dogs do not camouflage well in the dark and guard dogs make lousy pets. However, boxers are no longer guard dog material and pets have little reason to camouflage themselves (except when they chew up your favorite shoes).
I have yet to locate conclusive scientific evidence that supports the pro-culling claims of health risks, deafness and increased chances of being abused, neglected or otherwise mistreated. The only increased health risks so far as I can tell are skin irritation and sun burn. This makes sense to me (I'm blonde - I know sunburn!) but these are not insurmountable.
I've been told that no studies have been done on deafness in colored dogs, so I'm not sure what the "increased" incidence of deafness in whites is compared to. There is some evidence that deafness due to hair loss in the ears is caused by lack of pigment, so this type of deafness could be higher in dogs with white ears. This sounds plausible to me. I've not seen any stats on blindness.
I've surfed a lot of rescue and humane society pages over the last few months and it doesn't seem to me that the number of whites is disproportionately high. This is not scientific, only my observation. The number of people wanting to adopt a boxer in Canada seems to outnumber the number of boxers needing homes. That is encouraging, but doesn't tell me that white puppies specifically are doomed to a hellish existence of pain and suffering.
My point to all of this is that this practice does not seem to have a place in modern times and it is unfounded perceptions that perpetuate it. At the very least, it needs further study. We do live in a free society and this practice is not illegal, but like I said, I will probably seek out another breeder who does not practice culling.