I shun fast travel, however, where settlements are concerned, walking (or running) is not always possible. I have had settlements go under attack and received the "you have failed" message in a matter of 1-2 minutes. Not nearly enough time to travel via foot, or even vertibird. So, depending on the settlement, I will sometimes fast travel to a settlement when under attack. My main settlements have a high defense to attractiveness ratio, so this is not very often.
In a more general sense, I have taken the Skyrim cart approach to this. For any two settlements that are linked via a supply line, I will allow myself to fast travel between them, in the order they are linked. I figure I am just tagging along with the Provisioners and skipping the mundane.