I respect that caution, But something to consider. In addition to my friend, the former pro racer, the Girl Scouts also teach a certain amount of assertiveness. Yes, the Girl Scouts. We did a training with the whole family last summer sponsored by my daughter's GS troop. They explicitly counsel against staying as far to the right as you can. Because sooner or later, you are going to have to get left. And you may very well surprise a driver by doing that. Or they just assume you will yield to them. Or worse, they don't notice at all. I was very impressed with the training which covered all kinds of aspects of safety for riding on public streets, riding in groups, as well as basic agility and control drills. We'll probably do it again this summer as a refresher for the kids.
This and more. Here's the thing. There are a hundred and one things that can distract you and make you twitch in some trivial but unexpected way. A loud noise nearby. Chipmunk darting into the road. Rolling over a big pebble or a little divot in the pavement. Hitting a bit of sand or a damp patch of line paint. Fly smacking your glasses or bee stinging you in the thigh. The rider in front of you brakes unexpectedly. The list goes on and on. It doesn't happen on every ride, but it WILL happen to you, sooner or later. And when you twitch, you're going to go off your line. If you have left a couple of feet to spare, it will be a complete non-event. You will forget about it seconds after it happens. However, if you are too close to the edge your front wheel is going to go off the road and you are going to go down into whatever is there, or you're going to catch the edge of the pavement as you instictively compensate, and go down that way. Even if you don't actually go off the pavement, the fact that such an event is imminent may cause you to over-correct and skid out - possibly under an approaching car ... which would have been leaving you more room if you had been farther out into the lane to begin with.
I coach riders to stay at least 18" from the edge of the pavement at all times.