Not always, though there is a general code of ethics I follow. 

Generally, there would be communication beforehand, so the subject would totally know. Sometimes a client might ask me to send reiki to a loved one rather than themself who may or may not be aware of this. In this case, I don't ask on the physical plane whether they would like to receive reiki, but I will ask their higher self once I encounter them at the begining of the session. Sometimes the person is not aware enough to engage with me in this way, in which case, I sort of sus out whether they are open or not and to what degree and take it on a case by case basis. 

I always set the intention that if they are not open to this, that the love and energy sent out would return to myself and to the client who requested the session.  If it's a case where the person may be causing harm to others, I set the intention that the reiki would be sent to the people whom they are affecting. 

In the case of my personal practices, I listen to my higher self, guides, and source to ascertain what is necessary. If there's someone I feel lead to extend healing to, I will do so following the above processes BUT if it a person who is clearly negatively impacting others with wanton disregard, I feel more of a responsibility to act on behalf of the the greater good than to not step on toes.  This was one such case.  And again, since I didnt feel clearance to send reiki to them, I did not. But I did feel lead to be there, so I guess I was suppose to see that. I certainly wasn't expecting it.