Rule of thumb should be, if you have to move longer than 2.5 seconds, you should be using scathe.

A good example of this is Ravana orbs, or having to drag your add in A1.

Obviously use your procs if you have any, and then swiftcast if it's available. But if you have neither and you have to move for a mechanic that will take longer than a single GCD, you should be using scathe. I get frustrated when I hear ppl say it's not even on their hotbar