Blade centering is something that I pay a lot of attention to when buying a used knife and upon first opening any knife I buy. I'll be honest, It does bother me if the blade is not centered no matter how much the knife cost. I get irritated when a knife in the price range of a Spyderco,Benchmade, ZT etc. is off centered. It bothers me because I have had many knives that costs a lot less money that come with a nice and centered blade and I feel like it should be paid more attention to during QC. Also some companies will say an off centered blade is "within tolerances" and won't want to do anything about (From seeing others people's experiences, I haven't run into this situation).
That said, I've never asked to return a knife from a private seller because of an off centered blade. To me, it's my responsibility to ask the seller if the blade is centered and what the lockup is looking like. I've failed to do so for one reason or another before and suck it up because that's on me. And come to think of it, I've never returned a knife to a knife dealer of any kind because of an off centered blade. That's usually because I don't want to go through the hassle of sending it back, waiting for them to confirm they got it, then waiting for shipping again, as well as paying for said shipping. So it does bother me for sure, but I usually just suck it up and tweak on it until it's as good as that particular knife is capable of being. If the blade rubs on the scales, that really bothers me. However my current beater edc is like that and since it has battle scars anyway, it doesn't really concern me.