That answer varies from person to person. Some send their healing out to the universe for the person they are healing to be able to collect it at their leisure, or when they want it. Some work with the actual person at a designated time. I guess to them they feel it's stronger if both are receptive to each other at the same time.
The healing energies can cause a relaxing effect on the person. So some want to make sure it's not done when they are buy working, or driving, and prefer to do it an more opportune time.
i think it is a beautiful thing~to think of others and try to heal them. So is this where your spiritual interest originated? Is Reiki mostly for physical healing or does it also include mental and spiritual healing?