LM - my experience is from 1985, so things may have evolved a lot, but everything that I saw get graded for human consumption was 100% A-OK.
A lot of it was about the visuals. At the highest level, the fish gets removed from the line and specialist comes in and prepares it for a high-end restaurant at some multiple of cost. From there you drop down to things like large filets that would get displayed on ice in a fish market, then the filets that you would get in a plastic bag in the freezer section and on down.
The salmon that was starting to rot was graded for pet food and probably would not hurt you, it just wouldn't taste or look good.
Also, the plant was a pretty tough place to work for many reasons, but nothing I saw there seemed unsanitary or unhealthy or gave me any concern.
Granted, it was one plant and it was years ago. My observations may have no relevance to today. But if I were you, my big worry with salmon would be mercury, not the processing plant. Mercury is only a problem in high concentrations, but it is a consideration if you eat a lot of salmon. Guru/pitchman/whatever Tony Robbins famously got mercury poisoning when he decided to eat "healthy" and started getting almost all his protein from large daily servings of salmon and nearly died.
All that with the caveat that my one data point is Tony Robbins, who I don't consider the... uh... most reliable source on anything