I have a conflicted point of view on this. Sometimes it makes sense to keep in as much air as possible between mouth-breaths. Other times if I'm not careful it tends to raise the torso ahead of the hips; I'm less aware of my legs at that point of time, which likely means some relative sinking of the back end.
It works best when with a full lungful I get just the amount of downward angle in my lead hand spear, letting my head drop just a subtle amount and pushing my chest down, maintaining a non-exaggerated long straight body axis, and my legs seem to behave themselves gliding in my streamline shadow without too much physical effort. But it is difficult, and most of the time I don't get it. But at least I know when I'm getting it.
Other end of the spectrum was today -- our first wet-suit day in the pool for the year in our TI swim club. It was nuts, bouncing like a cork on top of the water, or at least that's how it felt. I had to consciously control my tendency to over-rotate. But once I got it, I was able to get crazy huge long strokes and glides. I easily got under 18 SPL (25m pool, 162 cm height). But now I remember last year even with a wet suit I couldn't get as low as this SPL number. Must be something to do with some balance skill and core control that I have acquired gradually over this past year. But now I know how "normal" buoyant people feel floating. OK, maybe, this wet-suit assisted float is unnaturally exaggerated, but without it I'm usually lying at the bottom of the pool. And absolutely no problem merely rotating my face slightly to breath.