Once my BFB get established, I cut them back to 12-15 inches every February. I like the look of them when they have more fullness at the bottom. My mom prefers to limb hers up more like a crepe myrtle. It is just hard for me to deadhead when the bulk of the new growth is over my head(and i am 5'11"). My white one that I had in Hillsborough would be trimmed back to 15 inches and would top out at 6 feet or so every summer so they still get plenty big even if you trim them back. I guess what I am saying is that I do not believe it is necessary for good blooming but trimming them back is an aesthetic choice for me.