I think a fair number of people have a slight cant that goes unnoticed.
Lots of people cant when shooting single handed, opinions vary on whether it is bad, I've known some LE that were taught to do it in their academy.
So far as national trainers Vogel I think said at the typical distances that one is required to shoot one handed it is not a big deal if it's minor. Stoeger will jump on you for any at all he is very accuracy driven and it will change your POI/POA relationship, more noticeably with distance, of course if you're shooting 6" groups you're unlikely to notice an inch or two shift. I prefer everything as near the same as I can get it all the time, so I hold the gun pretty vertical, I do battle a slight cant WHO, which is how I learned the depth of Stoeger's opinion.