I like that imagery better than keeping a foot on some fucking line. Plus using toe as a verb is dumb.

When I was a kid, the school gymnasiums had many different painted lines, delineating boundaries for basketball courts and other games. Gym teachers would often start activities by getting the students to line up on one of these lines, a position from which we could all see what the teacher wanted to demonstrate. We'd put our toes on the line. We would literally "toe the line." That's not a metaphor, not an imaginary image. I would guess common usage comes from the military more than from gym class.