Some claim you have to ask permission in order to protect the person's free will or freedom of choice. Well, that sounds nice but it is a rather moot, not to say pointless argument. Free will is an illusion, we face limitations and boundaries in every moment of this earth-bound life. In addition, what happens on the mind-level versus the energy-level can be, and often is, conflicting. A person might say they want it but they are far from ready on the energetic level; others refuse the offer even though they need it more than anybody else (i.e. people who are to proud to accept help).
Certainly, if you actually have to physically touch someone you should ask permission because if you don't, you might get punched. Energy work does not (necessarily) require physical touch, however.
If I'm inclined to work on someone I feel it out rather than asking for a literal permission because the feedback from my senses is way more accurate than the literal responses I might get. Do I breach through every boundary to get the work done? NO! If it doesn't feel right I won't do it; but if it does feel right, I will do it without "permission" and announcement.