As much as I like in doing that, in some locations it is illegal and signs are posted to that effect. I guess authorities figure your brake lights aren't lit up to warn those behind you that you are slowing down. I drive a couple stick-shift cars and downshift to slow but haven't been caught yet in those zones.
Braking to down shift was used a lot back in the day when transmissions were cheap to repair and brakes were expensive. Now it is the other way around.