2015 Chevy 2500HD 6.0L gas Ike Gauntlet

I have owned three Silverado's with the six speed transmission and all downshifted when necessary. When I approach a mountain grade, I use the M option to select a gear that can be maintained to the top. I have this theory that it is better for the transmission to begin the climb in the proper gear rather than make a full throttle downshift half way up.