We might consider differentiating between technique and crutch.
Bill's tip is an exercise in developing good technique - his use of the small adjustable square develops awareness of the connection between accuracy with chisel and final fit of tail in tail socket. My guess is that once the eye is trained to recognize what the check helps detect (incorrect angle of final cut with chisel), the check is likely more a case of comfortable routine than true necessity.
I view a crutch as something used in lieu of further development of the skills or technique necessary to achieve the desired outcome. The setup pictured is perhaps more of an aid to development of a sense of square for chisel use than a crutch, although I suspect that prolonged use might not yield the same results as Bill's post-execution check of results.