I don’t mind them. Where I do grocery shopping there aren’t any, but even if there were, I’d still prefer going to a cashier in fear of messing up something. Even if I’m somewhat introverted, saying “Hi” and “Thanks, have a nice day” isn’t particularly hindering. As for whether they’re job killers I’m not sure; at that same place employees that are doing check out are doing everything else around the store as well.