i've only ever done bevel-up. never had a reason to try bevel-down, so i really can't give an assessment of why it would work better than bevel-up.
i would venture to say that since you are sticking with ultrasound, and are using the us image as your guide as to when you have gained access into the vein, then it probably won't matter much.
as with so many other things, it probably has much to do with what you are comfortable with.
i had one of my coworkers try to get into an argument with me one day over what wire is best. i didn't agree with her, but i refused to argue. the bottom line is that wire a works best for her; wire b works best for me.
my point is that i suspect the nurse that does bevel-down (which is "unusual," but not necessarily wrong) starts pivs that way for one reason or another, and that is what the nurse is comfortable with. so that is the way that nurse starts piccs, as well.
i know, i know.......so many words, and i didn't even answer your question! sorry.