Some resources on brass instrument technique recommend pushing the lower jaw forward such that the front teeth are aligned. Is this a controversial recommendation? If not, how important is it to proper technique? As a trumpet player I've noticed that doing so helps with aiming the instrument upwards relative to line of sight. Are there other advantages to playing like this? Is it generally considered worthwhile, as an intermediate trumpet player, to work toward this posture in the jaw if one is used to playing with an overbite?
This is a personal thing and there's no universal answer. I play trumpet with a slightly upward angle, and I know great players who play with all sorts of angles.
That being said, it seems like most pros do push their jaws out. Doing some Google image searching, you'll see that top players usually play straight out (Wynton Marsalis), upwards (Wayne Bergeron, Arturo Sandoval), or at most only slightly downwards (Roy Hargrove, Allen Vizzutti), indicating that they've moved their jaws forwards. My understanding is that this makes the mouth larger and helps create a more open sound. The only pro I can think of that plays with a significantly downward angle is Sergei Nakariakov.
As a trumpet player I've noticed that doing so helps with aiming the instrument upwards relative to line of sight.
Where the horn is pointed doesn't really matter, it's usually easy enough to point the bell wherever it needs to go. In my personal experience, my upward angle has been a detriment a few times: In marching band for matching horn angles, and in some specific scenarios when I've needed to point the bell down for acoustic reasons.