Do you ALWAYS need to ask permission - NO
When to, when not to, what are the deciding factors?
Here's my rule of thumb:
Just Do It!
If you are clearing entities or dark energies, the answer will never be a no, so just do it.
Personal healings or things that involve free will of the human or the path of the Soul can have inappropriate timing, because sometimes there is a lesson or awareness that needs to be internalized before the change is implemented either for the person affected or someone around them.
Jumping the gun on this will just put them in the position of having to learn it another way and perhaps in a more harsh way.
This isn’t rocket science to determine. You’re a witch!!
It’s easy to know!
If you dowse, just ask whether you should get involved at this time or whether the person is ready to receive a healing.
The only real advantage to this is to know whether the healing or action will be immediately activated so you can be watching for the result. Otherwise, just get the ego (the need to know the results) out of the way and put the action or healing out there into the universe or aether with the intention that it be waiting and available to be fully used as soon as it is appropriate for that person to receive it.
Trust in your power and the wisdom of the Universe. Go ahead, do it!