I guess I don't completely understand your question, meaning that if you recompose you wouldn't have "headroom" at the top of the frame ?
I go with two different methods in continuous mode (using AF-ON to focus). The simplest, being, focusing (let's say on an eyelash), recomposing without focusing, tripping the shutter !
Or I go in 3D 51 mode, focus (same eyelash, with good contrast), recompose still focusing (following the red square moving to stay on the eyelash, if it's still in the AF points array), then tripping the shutter ! What works for moving subjects, works also if you are moving the camera