Why wouldn't that same technique apply to the .38 S&W? It mimics loading the case with black powder and the .38 S&W was a black powder round.

I think a 'good Trail Boss load' would be just what Smith Crazy explained: Fill the case to where the bottom of the bullet will be when crimped.

However, this might answer some questions, too:
38 S&W Trail Boss load data needed
http://hodgdon.com/PDF/Trail-Boss-data.pdf