I'm always for killing every walker that you have a chance to kill, it's just stupid not to! Even if you don't think, and think being the operative word, that a walker is dangerous, it could still kill someone.
People keep saying, "It's just one walker..." But no, it's one or two walkers at a time. They have had opportunities to take out walkers and just shrug and decide it's not worth it, an attitude which has killed two characters in the series already. The fact is that every walker is dangerous and can spawn new walkers, which makes eliminating them when possible a necessity for survival. This attitude of 'Eh, it isn't a threat to me' is really aggravating, because I can see how nearly every character has done it at least once. How many walkers have they ignored or created because of this arrogance?
Also: As for Hershel teaching Carl control, I see this scene as doing the exact opposite. Carl already knows that Walkers are dangerous, he knows the difference between life and death, and he known the consequences of not taking action when he should. Dale died because he didn't take care of a walker, and that is not something he is capable of just shrugging off for plot convenience. By telling Carl to ignore walkers, Hershel isn't teaching him to value human life or to assess risk, he is just proving why it is they need a more practical view on walker extermination.