Perhaps my initial question required a greater introduction.
During my own training I was adamantly told that straight and level flight means:
(five criteria)
wings are level,
constant height,
constant heading,
constant airspeed, AND
ball is in the center.
However, I always maintained that if the aircraft is ACTUALLY flying with wings level and at a constant height and heading, it would be balanced by definition. That is, if the ball is not centered in straight and level flight, one of the first four criteria is not being met. I'm not even sure airspeed is strictly a consideration in S&L flight.
Is the classroom definition of S&L flight most simply:
Constant heading, constant altitude, and in balance.