I never really thought too much about it. When I was learning to Fly Fish, they said to have it just tight enough so that it doesn't free spool when you pull line off.
Like some of you, I do tend to adjust it during the fight. If it's running too easy, I tighten it up. I also find that when I have my rod under my arm, the drag gets loosened from flopping around under my arm that I have to re-adjust when I am done tying my flies on.