score:4
In order to determine this, one could ask:
Hopefully, they will tell one to go to a giftshop or something like that. Or they may inform one of something like the special hotel in Berlin.
If one got a yes from all these, then I suppose one could go ahead, but only in the exact manner advised.
If one got a "no" from any one of these, I would advise against doing it.
I want to say that I believe it's extremely unlikely that one is going to find a historic landmark where such people will tell one to go ahead and chip off a piece for oneself. If one is looking for that, one will be wasting one's time the vast majority of the time. It's better to go and enjoy one's holiday than to waste one's time in such pursuits.
Moreover, while I understand that some people might be, I would not personally be interested in chipping off a piece of a monument, even with explicit permission. I believe that the beauty of a monument is in the very location, not in taking pieces away for oneself. However, that's beyond the scope of this question.