The only time saying "ks" can be acceptable, but even then still discouraged.

Is when someone arrives in your lane out of the blue and last hits someone that you were undoubtedly going to kill in the next second or so.

Even then it is still discouraged.

1. Even if you get less gold for just an assist your team generates more overall gold. 1.1) If it was the support or jungler that got the kill, especially in the early game, that gold can be a lot more useful in their hands for wards, boots, finishing the jungle item, etc., all of which don't come that fast when you're not actively farming creeps in lane.
2. Either way, your opponent is going to get behind in cs and overall game presence.

So stop crying that someone stole your kill, especially when they did most of the job anyway.